How many spells do you expect to cast in a day? The number of spell slots went down only a little from 2e. There are still more than enough slots to cast only slotted spells at higher levels.
This varies greatly from table-to-table. We've had plenty of adventuring days were our casters ran out of spell slots (well spell points now...), and it wasn't because they weren't being played well... If the number of slots fits your table's style, that is great. We find it lacking at times, personally. Switching to spell points has made that a bit better and really focuses your resource management IMO.
And I see you completely missed my point when giving the examples of spells. Like I stated before, they did the scaling the way they did on purpose, so even though it's an option to cast a lower level spell at higher levels, they make it so that you want to cast the stronger ones instead.
Your chances of rolling all 6s on 3d6 is a lot higher than rolling all 10s on 3d10 or higher, the chances of even hitting only 2 people with does more damage than firebolt up until level 17, and if you're a smart wizard, you can find situations to make spells like burning hands more advantageous.
No, I saw your point fine but don't agree with your analysis. I don't care what the chances of rolling are on the dice, I am working with average or expected results. And again, playing the "if you're a smart wizard" line is only making you appear less worth my time. But for the sake of discussion...
A wizard should always look to use the most advantageous spell in the situation. We have many times when our table upcasts Magic Missile over casting Fireball. Why? Because there is often just one target and the expected damage for 5 missiles (17.5) is better than the expected damage vs. a saved-against Fireball (14). If the Fireball can be cast without risk to allies, we do that instead. Of course, if the target has resistance to fire, Magic Missile is superior again. And so on...
The difference between 1e and 5e is that in 1e and 2e, when the cantrips were level 1 spells, they had to automatically scale because you couldn't cast spells at a higher level, you had to prepare the same spell multiple times a day if you wanted to cast it more than once, if you prepare a spell you didn't use that day, it was a wasted slot, and if you ran out of spell slots, you were completely defenseless. Also then came the balance issue, where the wizard became so much more powerful than everything else at high levels.
Cantrips gave a wizard viability and survivability over those previous editions without breaking the game. If you're old enough to have played back in those days, then you should have recognized this.
Cantrips were level 0 spells in UA in 1E. I can't speak for the change to 2E, I honestly don't recall and would have to dig out my books to verify.
You also didn't have to prepare spells as higher level, the autoscaled. A Fireball in 1E did 1d6 per level of the caster. Period. IIRC 2E limited it to 10d6, but in 1E there was no limit.
The LFQW issue so many people seem to have was never an issue for our 1E/2E games. Wizards were super powerful, but spells could be disrupted by attacks (not in 5E) and when you did run out of spell slots, you
were screwed unless you had powerful magical items to back you up.
Any power issues were balanced out by the fact that magic-users (let's call them what they really were in 1E

) had to choose spells for their slots, preparing a spell multiple times, worrying about wasted slots, etc. You want to talk about having to play your magic-user intelligently? 1E was all about making the smart choices for spell selection and using those spells wisely.