Why is most of the stuff in the marketplace not compatible with unreal engine 4.9?

I am sorry but to avoid all of this couldn’t you just add backward compatibility to Unreal Engine itself? So that assets/plugins that don’t get updated would still work “automagically” (a word known in the SCSI world) ? Of course most or none of the new engine features wouldn’t be used but at least the assets would still be usable without modifications even with a performance hit due to the obvious internal translations/mappings you would have to enable to make those work when something has changed.

Yes you would have to add some new layers of code in order to make such a thing work but you would avoid having to break compatibility for both projects and assets in the marketplace between new Engine versions.

If you keep releasing so many new Unreal Engine version each year that change critical things so much you end up having to do a lot of extra work yourselves and oblige both developers and marketplace assets developers to do extra work and extra checks on everything as well.

Adding a translation/mapping layer to ensure some backward compatibility would ease both your life and that of your customers. Or not ?