I'm speaking from experience, so you are just wrong about that.
Many games, and WotC products, go through the fist couple of training wheels levels quickly. The House of Lament in VGR takes PCs from 1st to 3rd level in about 3 hours, then 4th at the end, after another 2-3 hours play.
Can we at least acknowledge that your expeirence is abnormal? Because I've been in a lot of games where level 1 took a session, level two took a session, level three took two sessions, level four took two sessions, and level five took three. Which is 9 sessions. Not two. I don't think I have even heard about people going from level 1 to level 5 in two sessions before, if you are going to do that, just start at level 3.
"The squad of skeletons marches down the passage towards you."
I've seen 12 enemies taken out by a lightning bolt, although you are more likely to get more targets in a cone. Whatever, so long as you can hit more than one, it's better than using your sword. If you have one enemy, use your sword, that's what "situational" means.
And I once saw 50 enemies taken out by a lightning bolt (skeletons funnily enough) but it was a massively contrived scenario. Also, there is a bit of a difference between a 100 foot line that used to bounced off walls and a 30 ft line. It is about 70 ft.
Additionally, your phrase could be interpreted many ways. Is the squad of skeletons coming down a 5 ft hall or a 20 ft hall? Are they jammed together or are they walking with a 5 ft space between them?
And is it automatically better? I'm just going to remind you that 2d10 is 11 damage on average, where swinging twice is on average going to be 2d8+10 or 19 damage. That means that if both enemies fail their save, you've only done 3 more points of damage overall, but if either of them make their save, you've done less. And, I've definetly felt first hand and seen first hand that dealing 5 damage to enemies at levels 8 or so does not feel powerful, it feels like a waste, even if mathematically you have done more damage, because injured enemies are still 100% effective in combat.
Here is a tip: don't choose lightning or poison breath. Not all damage types are equal.
Not everyone makes choices on their race based on what is the most optimal, and it is a bit disingenuous to say "This is far too powerful if I ignore all the downsides and pick the best possible option" After all, Fire and Cold resistance are more common than Lightning resistance, so you've essentially gotten to "just pick acid"
No DM is going to have you fight damage resistant enemies all day long. The fighters would get really really pissed off. You are going to want to use your breath weapon several times in the boss fight, and not at all against trash mobs (unless you are in a position to vaporise a small army), which is why PB per long rest is so much better than once per short rest.
Wow, condescending and wrong. First off, let me point out that any DM who ever had their players go through Descent into Avernus has had their players fight enemies resistant to physical damage all day long. In fact, it can be rather common when fighting at certain levels or against certain categories of foes for that to be the case.
Also, you have no idea if they are going to want to use their breath multiple times on the boss. In fact, your framing of never against mobs of weak enemies and only against bosses wastes the only actually powerful thing about this ability. I might agree that it is cinematically cool to breath fire on the boss during the final showdown, but this is an ability designed to be effective at hitting multiple enemies, not single targets. And every player is going to recognize that.