The flexibility of changing your spell list. What’s the point of a Cleric or Druid being able to learn any of their spells if they never change their spell list at all? It’s just an extra consideration (“Should I switch this spell I never use?”) that adds to the mental weight of playing a caster.What flexibility are you referring to, can you elaborate? The neovancian(?) Style used in 5e does more to eliminate the opportunity cost of preparing a niche spell over some other spell or having some allotment of a given spell instead of a different allotment than adding flexibility. Scrolls were always a thing for flexibility, they just had a cost in terms of time gold and sometimes container interactions or DC reductions from using a scroll instead of slot.
Ultimately, I just don’t like Vancian casting, especially the old school one. It’s needless complex for what amounts to dropping plot coupons. It only feels ‘magical’ if you’re used to it, otherwise it’s just a long series of arbitrary rules. There’s tons of games that have different magic system and, at this point, D&D just sticks with their Vancian variations just because people expect it.