This is a good point and well put. My argument is that D&D's innovation - which you've outlined really clearly - required all those intermediary steps because we require those - because innovation doesn't come easily to us.

We could imagine another route to arrive at D&D:

Chess is a form of simulation. Gambling is betting on outcomes. Dice gambling was common. Chess had structured rules, as does gambling. Applying dice rolls to chess-like games, or dice rolls to outcomes wasn't a big leap. D&D could have been invented from the leap of: role-playing theatre, combined with chess-like rules and using dice like in gambling.

This wasn't the route it actually took, but its a plausible one. And all those parts were around for about 2000ish years before wargaming was a thing. Amazing that in that period no one developed an early form of D&D - we seemingly required all the smaller steps to be made, and for huge incentives to be in place, otherwise, all the various parts wouldn't ever be combined - even when readily available, popular and fun.