1. One of my games started at level 12. It's fine.
2. Why would we not be leveling our characters' features as we level??
Not leveling of characters.
Adjusting of characters' abilities as they level.
AKA a swing swapping out Burning Hands because it no deals less damage than a Cantrips for Silvery Barbarian and Shield and then every single turn is "Do I can SB?" or "Do I cast Shield?" or some other 1st level spell that has it's rules reread during play.
The slowness of turns, swinginess of power, wrecking campaigns, and wane of death usually doue to this.
I've been playing D&D since 2000 and the biggest turning point of ugh in D&D is when spellcasters realize their low level spells aren't cutting it for damage or range and swap them for utility spells, support spells, control spells, and OOC spells.
When the wizard, cleric, druid, or whatever starts casting 1st and 2nd level spells you don't see often or they are used to, it's big UH OH time. Worst if from self crafted downtime items.
And when experienced, big buyer, players
start at high level, they tend to
start at that point.