Thank you for the project and updates !
As Blender 4.2 LTS is a Long Term Support version i think it will be a reference for future extensions with the blender_manifest.toml.
Of course if it is possible the ideal is to support both the new extensions and old add-on system but if resources and time are limited, in my personal opinion it make sense for a “new” framework to support more the “new” extensions and less the “old” add-on system, also because Blender 4.2 LTS is planned to be released on July 16, 2024…so in few days.
To be clear, for what i understand, Blender 4.2 LTS will continue to support also the “old” add-on installations.
There is an ongoing thread:
and video: