I realise that your numbers are only meant to be illustrative, but it's worth realising that this is "only" 65 hours a week for 40 years. A lot? Yes. More then I'd be willing to work at a typical desk job[1]? Probably. But feasible for a passion project? Definitely! (and doesn't even necessarily require starting before you hit 25, although that clearly helps...). The limiting factor is not so much that nobody can do this, but that (short of independent wealth, which I think can bring its own set of constraints and expectations) very few people ever get the opportunity to do this without a lot of interruptions. I don't think it's impossible to imagine a society where that isn't true.

[1] although... "thinking" can stack surprisingly well with some activities we'd consider leisure, like walking or (at least in my case) gardening. So maybe this isn't really going to be 65hrs/week at a desk...