I think Apple would want to release a 64GB or 128GB Mac Pro... but only to upsell its customers to a 256GB or higher Mac Pro. The ability to configure more RAM into a system is key for marketing (even if this RAM is soldered onto the motherboard: like LPDDR4x, it wouldn't be too expensive to make different motherboards especially in a niche $10,000 computer like the Mac Pro).
But if Apple did go with HBM2 itself... well... then its physically unable to go above 32GB (today), or maybe ~64GB (if bigger HBM2 modules come out next year).
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Sticking with LPDDR4x just seems like the most obvious, and best, decision for Apple. So that's my expectation. But then again: making a 8-way decoder + fat-as-heck execution pipeline wasn't part of my expectation either. So maybe Apple will surprise us again.