The limit is strictly to cost of memory at this time. If you wanna get by that limit, you have to, well… design it yourself in house.
Spoiler alert: they're supply-constrained too. Their customers will pay to outbid Apple, just like how Apple outbid the industry on TSMC's 5nm node.
If Apple can't place DDR5 orders that outbid the HBM demand, then yeah, Apple Silicon is going to get fucked over by every vendor. They'll be laughing their way to the bank while Apple begs Californian courts to give them free RAM.