Just wanted to cosign that this is absolutely a real-world concern. For that reason I always try to make huge reservations as early in the process lifecycle as possible [1] and then subdivide them from there. Then you can know ahead of time how much space you have to work with. This still has potential composability issues, but partitioning the address space like this before VA fragmentation becomes an issue really helps when you have a greedy VA reserver competing against typical workloads (and their malloc's mmap/VirtualAlloc requests), especially in long-running programs.

[1] Ideally before most shared libraries have been loaded. I should do a simulation to get some quantitative estimates, but my hunch is that ASLR hurts badly even if each loaded library only takes up one 4 kB page. E.g. if you have an empty address space, allocate a 4 kB page at a random address and split the space in two around it, then try to densely pack as many 8 GB ranges in there as possible, you lose one potential 8 GB range (even if the ranges only require 4 kB alignment for their base address). Whenever such splits don't land in the same 8 GB clump, you lose a potential 8 GB range per split.