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.
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.
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.)