pavlov and modern psychology would like to show you that irregular rewards that can't be predicted don't really motivate people.
Er...that's not exactly true now, is it? Making it so the rewards are semi-random, rather than
completely unpredictable OR completely predictable, actually motivates people just as well, and sometimes
more, because they'll hold out for a benefit if they think it's coming.
Wikipedia: Intermittent Reinforcement Schedules.
If it were up to me, D&D would have a 30 level standard. 5 level for each tier of play.
1st level would be a true "zero" but you would not be expected to start at level 1. The 1st level 5e PC would be a 5th level PC in that game. 6th level is the beginning hero out of apprenticeship or training.
This gives you more levels to fill out class features for while offering that "zero" play of a wizard with one spell or a fighter with weak equipment.
That's a True Issue with 5e.
Whereas for me, I would also do 30 levels. But you start at level 1.
All the "you are not an adventurer yet" would be handled by distinct "zero level"/"novice level" rules, which could be theoretically extended, if not totally indefinitely, then to a pretty extreme degree. That way, folks who really really really love the "zero" end of Zero to Hero can stick with it for a really long time, while others can move past it if they wish, and still others (read: most folks) can skip it entirely, as is done in a lot of fantasy fiction.
you have story advancement in D&D too, every adventure and adventure path has goals you reach by completing it and probably also along the way (depends on the length of the adventure, always true for APs). The two are not mutually exclusive
If all you cared about were leveling, you could fighter ever stronger opponents in an arena every day, yet no one ever does that
Conversely, if all people cared about was these major goals, you wouldn't have levels at all, you'd be like Conan. Clearly, we want something in-between. Some folks find the early-level grind, where you're sharply limited in what you can even attempt, to be AWESOME and challenging and invigorating, and get bored when the
attempting part isn't what is a challenge. Some folks find the late-game plateau, where you can be pretty confident your
attempts will work, but need to put those things together in the right ways to reach a greater goal, to be AWESOME and challenging and invigorating, and get bored when the
attempting part is a total slog.
Hence, the best way to approach this is to write rules which fully support
both things. Zero-level or "Novice" rules that allow those who want to spool out the "learn how to even
attempt to do things" phase. 1st-level rules that make quite competent, but not yet diverse or truly "powerful" characters yet. And high-end/"epic" rules that truly transcend limits and put the focus on things like collateral damage, narrative/personal achievements, and other non-measurable sources of meaning.
Instead, we're left with a sprawling mess that forces newbie players who haven't a clue into exactly the same risky, dangerous, lethal levels that are meant to appeal to long-time fans. Exactly the things that should be hooking people on the game require
extremely careful, kid-gloves handling to not drive them away. (And, before anyone asks:
yes, I have absolutely played in games that drove brand-new players away from 5e permanently.)