PY = MV, but we haven’t discussed V at all yet.
The argument that fixed M and rising Y must cause deflation only holds if V is assumed to be fairly constant. Otherwise an increase in Y could be accommodated by a corresponding increase in V.
Constant V may have been a good assumption for fiat back in the mid-20th century (when a typical dollar could take a week to cycle from bank to wages van to pay packet to purse to shop till and back to the bank) but it may not be so good even for fiat today, let alone for crypto five years from now.
I sometimes wonder, if a cryptocurrency finally achieves real-world transactional use at scale, might its V grow faster than its Y so it starts decreasing in value?