Earlier quoted context omitted.
64GB is available on the current-gen 64GB, so not offering it would be a step backwards.
64GB was available on the previous Mac Mini, but they took a step backwards and capped the new one at 16GB, no reason they won't do the same with the MBP.
From my experience with both, I think you might be very unhappy in retrospect if you bet that more RAM is going to make the most recent Intel one come within shouting distance of even an 8GB M1 Mini. The old one was one of the best for single-core performance within its generation, but they're not even the same thing, not even close. Again, I own both machines with the Intel Mini maxed out as far as it can go with RAM.
My gut level reaction is, you could give the new one lots more RAM (in theory) and it wouldn't end up using it. So they're capping them, otherwise people will assume there's a benefit and that the 8GB and 16GB ones aren't workable for heavy processing.
I've also got a suspicion (possibly just a weird cynicism) that this is why the previous-gen Mac Pro runs so huge and expensive: they've overpriced and overdesigned it so hard that it's unattainable for nearly anybody, making it a sort of status object with Veblen pricing, specifically to avoid class action lawsuits when they release newer machines that outperform it at a tenth of the price.
You were never supposed to actually buy the monstro cheese grater Mac Pro or its screen with the thousand dollar stand, and if you did, (a) it's of symbolic value to you and (b) it's your own fault. When the real high performance computer comes out of Apple it'll be at Mac Mini prices, and meant to iPhone-ify desktop computing as a category.