Probably as bad as blocking a php-version for business reasons....
I repeat what I have said before, for us is the good strategy add feature to the last update and stable plesk version and use time of team plesk developer to still working on updating version and not waste time in adding feature on old plesk version, we are always talking about new feature, not critical bugs, it's something very different. I am not seeing an business reason on this, price is the same for onyx and obsidian.