In reply to David Finch.

David, I am not sure which is worse: Telling your clients that there will be an upgrade "if they want to do it" that will cost 20 billable hours, or dinging them for double that each year mitigating the problems caused by minor updates that change, break, or otherwise make wordpress not behave exactly as it has in the past.

I am not, btw, suggesting a totally new incompatible platform. Same house, perhaps with better plumbing and with the older code retired, new features added, etc. The other part is about establishing a stable supported version at 4.X so that if your clients don't want or don't need the 5.x feature set, that they could remain on 4.X and continue to receive security updates as needed. It wouldn't be a development environment, but would provide a proper end point for users and sites who need exactly that - a stable environment. WP could then support 4.X for a period of time, say until the release of the 6.X series in a couple of years.

It would certainly allow everyone to work to move forward in a nice solid leap, rather than shuffling forward until things break.