I generally agree with your assessment, but for what it's worth more recently Amazon has shipped its Elastic Filesystem (which exposes NFSv4) and Microsoft offe

I generally agree with your assessment, but for what it's worth more recently Amazon has shipped its Elastic Filesystem (which exposes NFSv4) and Microsoft offers Azure File Storage (SMB 3.0). AIUI neither is truly POSIX compliant but both are probably good enough for realizing most of the dream? I haven't actually had the occasion to use either of them in a project but very curious to hear about whether they have successfully enabled some useful legacy->cloud migration scenarios.