Could you borrow a bit?

32-bit platforms regularly use the entire address space (which makes sense if you think about it – it's far from rare for a program to want 4 GiB of memory). As for 64-bit platforms, they've been running low on spare bits recently: x86-64 and ARM64 can both (nowadays) be configured to leave some bits reserved for software and not use them for pointers or other hardware-specific metadata, but in this mode x86-64 has only 6 spare bits when using 5-level paging, and ARM64 has only 4 bits spare when using MTE. So there's theoretically enough room on the two most popular 64-bit platforms at the moment, but it's already pretty close to the limit and it's plausible that more bits will be needed for hardware features in the future (e.g. ARM64 used to have 8 before MTE was invented, and still does on non-MTE platforms).