The PCI resource allocation code is fairly intricate and everyone is scared that changing it may cause regressions. Sergei's patch set is quite intrusive and it would be necessary to somehow break it up into smaller pieces that are slowly fed into mainline over several release cycles, always watching out for regression reports. So, the problem is known, but the engineers working on PCI code in the kernel are given higher priority stuff to work on by their employers, hence the issue hasn't gotten the attention it deserves.

Actually I forgot to mention there's another solution: A PCIe feature called Flattening Portal Bridge (PCIe Base Spec r6.0 section 6.26). That was introduced with PCIe 5.0. It's more likely that FPB support is added in mainline than the pause/unpause feature. It's supported by recent Thunderbolt chips and it's an official feature of the PCIe standard, so companies will prefer dedicating resources to it rather than some non-standard approach.