The part you and many are missing this that 1 metamagicked spell is often better than 2 normal spells.
A twinned chromatic orb deals 6d8 (2 orbs of 3d8) in one turn compared to 2 normal chromatic orbs in half the time (1 round vs 2 rounds) for half the slots of a 1st level spell.
Think about it. A level 5 sorcerer has 5 points and 4 1st level slots. A 5th level wizard has 4 1st level slots and gets 3 more form Arcane recovery. The sorcerer can twin 3 of his 1st level spells (effectively 6 1st level spells) and still have 2 points left over. They just get the 3 extra spells and take twice the time. You get similar results with heighten and quicken spells. Distant spell saves you HP with range. Extend is good for buffs. Subtle is for low combat campaigns. And none of that 5th level or level restriction restriction of Arcane Recovery.
Metamagic is way better than Bonus Spells per day as it is often cheaper and faster. That's why you can't give sorcerers many bonus spells as they get even more stronger than normal caster with every metamagickable spell they get.
If Sorcerers had Widen Spell, people would be calling them broken. Widen Spell was 3 slots higher in 3E. Imagine if a sorcerer could double the area of any area spell for 3 sorcery points. Widened 9th level spells?
Most people don't realize how close to absolutely broken the sorcerer is. How bad it would be if you gave it the certain spells or gave it too many extra metamagics or sorcery points.
I completely agree that twin spell is very powerful, but that only hides the problem to an extent.
Let's say you take chromatic orb and twin spell. You get 2 more 1st level spells and 1 more metamagic. What do you take?
Let's look at metamagic for a second. Keep in mind this second choice is locked in and you will not get another metamagic until 10th level.
Quickened Spell is awesome, not a bad choice. Being able to bonus action a spell is incredibly powerful, allowing you to follow with a cantrip or do some other important action that turn.
Careful spell? The ability to deal half damage to allies if you throw an AOE on them, or avoid the effects of... actually at low-levels to sorcerers even have AOE spells that require a save to avoid effects? I can't find any since sleep and color spray have no save. Might have missed something, but that is a very specific build at that point, useful but may not be viable at low levels.
Distant spell? Great for turning touch spells into range, but not really useful otherwise. Average spell distance I've seen is 60 to 90 ft, and that is more than enough with an average movement of 30 ft on top of that. Could maybe be useful, can get ridiculous if stacked with spell sniper, but not generally useful in your average game.
Empowered spell is decent, odds of maybe increasing your damage, but it works best when you roll a lot of dice, which you won't at low levels. Also, specific to damage, so not useful in non-combat situations.
Extend spell? Double duration could be interesting, make mage armor last a whole day with a single casting or extend some illusions, but fairly situational. Making sleep last 2 minutes instead of 1 is fairly pointless, as after 1 minute generally you've mopped things up. Fog cloud for 2 hours instead of 1, potentially useful but only in very specific situations.
Heightened spell? Powerful, but incredibly expensive until you hit level 9 or so, otherwise you devote almost all you points to whatever one spell you want to succeed. With a decent charisma your save will be fairly high, and unless you are going against an enemies good saves your likely to get the spell off anyways. Still, if your build is built around debuffs this is a good way to increase your likelihood of success. Increase, because disadvantage is not an auto-fail, but you've used all your points either way.
Finally subtle spell, which I love flavorfully but can't see it being used much. I guess if you are enchanting and don't want them to know you spelled them, which they will anyways once the spells wears off, but it could be useful for casting in the middle of a fancy party where you don't want to draw attention. Or if you are tied up, but not blindfolded. Honestly, it is the most situational metamagic I can think of. Decent in the right circumstances sure, but those same obstacles could be overcome in manner different ways without using such a precious resource as a slot for metamagic unless you were certain of a concept or campaign where it would shine.
So, out of all those options... Quicken is clearly the best to me. So a sorcerer picks quicken and twin as their metamagic. Heightened and Empowered might be my high level picks, if the game or character get that far, but none of the others really measure up.
And yes, anyone can come up with a specific situation where extend for example is wonderful. But that is one encounter in 7 levels of gameplay. A sorcerer needs to rely on their metamagic options from 3rd to 10th which can be entire campaigns. More than likely you will be fighting during that time, and quicken and twin are also beautiful options for buffing sorcerers. But that means that realistically you do not end up with any of the 8 options, you end up with those 2 options, almost no matter what.
Then look at spells. At 3rd you will have 3 1st levels and 1 2nd level. What do you pick? If you went with chromatic orb, and we'll say dragon sorcerer so you don't feel tempted to take mage armor to increase your pitiful AC, what do you get? Shield is a good reaction to save you from a bad blow. Fog Cloud is good for infiltrating or running away. Expeditious retreat is better for per running speed. If you want damage other than chromatic you might take magic missile or ray of sickness. Comprehend Languages since you are likely the face? Detect Magic? Charm person or Silent image are essential if you have a theme other than damage. I could go on and on. You get to pick now, and while you could grab more or different 1st level spells later, you can only change spells once per level up.
At this time Divine and Primal casters get all their spells, just need to prepare what you need for the day. At this point wizards know 10 spells, two of which can be 2nd level. 10 spells known vs 4. Even larger gap if the wizard ever gets to stop by a library to scribe some spells into their book.
At level 20, if a wizard has never scribed a single spell into their spellbook other than at level ups during their entire career, it is 44 spells know to 15, and the wizard can likely choose 25 of those to have ready for the day.
If we agree that options equal power, then can you honestly say the ability to cast twice as fast can equal up to knowing at the very least 3 times as may tricks? And being able to cast them without spending spell slots if they happen to be a ritual?
Sorcerers dominate on the battlefield, where their speed casting excels at ending the fights sooner either through spreading debuffs or buffs more quickly or novaing damage like no ones business, but outside of that speed a wizard has them beat in every other category that a general, long term campaign will have. I see a sorcerer's power, I do, but it just feels like you have to play a sorcerer one way, as a great combat caster with only these specific abilities, and that can be very frustrating when you want to step outside of those bounds. Then to see the other classic casters getting so many nice things. Bards healing and inspiration is phenomnal in or out of combat. Clerics have no issues in or out of combat. Druids are insane, especially wild shaping druids. Even Warlocks get a lot of potential utility and crazy combos, like being able to naturally detect magic, which people who are born magical cannot do without spending spell slots.