Offtopic but, aren't you scared by "GitHub improves its PR workflow" and put your product out of business?
However I think the biggest issue with the landscape for code review tools is that 99% of developers use the default system that ships with their VCS. So on most teams, that's GitHub. People should be actively choosing their code review tools just like they choose their VCS, IDE, CI/CD platform, Issue Trackers, etc. It's one of many tools that makes up your SDLC "Stack".
As long as CodeApprove and Reviewable remain niche they've got nothing to worry about.