PHP6, similar to ES4/ESX, was a language version that ended up in spec. hell. Nothing to do with release woes (none were ever released), so I don't see any real relevance in this discussion?

PHP7 on the other hand was a pretty seamless migration from PHP5, and PHP8 looks likely to be similar.

General point is the original commenter was posing this as some fundamental issue of open source languages: clearly there's plenty of examples of success, so it can't be.