The problem is, those addresses are completely interchangeable, nothing stops e.g. malloc() from allocating addresses somewhere around the very top of the legal addresses instead from starting near the .data's end. In fact, it seems that mmap(3) in general does pretty much that by default, so reusing address's top-bits is inherently unreliable: you don't know how much of those are actually unused which is precisely the reason why x64 made addresses effectively signed-extended integers.
You opt-in to any of the top byte masking schemes via prctl on Linux. It's fully forward compatible, in that programs that don't enable it will continue to work like normal. Additionally, Linux won't map memory at addresses higher than 2*48 by default either because non-hardware accelerated top bits pointer tagging would have the same problem. I don't think either of your complaints are valid here.
