I agree with @hvelarde but lets put it in a different way.
Big disruptive changes... In core or out of core matters less than, does it solve all the usecases, or does it fix half the things and break another half?
For example to go all in a grid system like mosaic and switch to folderless and components, we need to ensure
- those that have rigid sites are happy (ie where they design layouts they don't want users to change),
- those that want their users to change layouts are happy (ie doesn't break in strange ways creating support issues)
- those that have different grid systems like bootstrap or css grids are happy
- those that don't want to use a grid system at all are happy
- all the plugins that assume a fixed layout are happy
- plugins that need default folders and portlets are happy
- core team who don't want to support different JS frameworks are happy
Instead what happens is we wait for deco, mosaic, pastanga, angular, react, vue-angl-act or whatever is the next big bang and be disappointed when it either a) makes things pretty but doesn't improve how plone works (ie the toolbar) b) requires a lot of upgrades and doesn't solve all the use-cases because it was created with a limited set of usecases in mind.
I don't agree with keeping things out of core that long term benefits everyone.
If either of the following are true then the core needs to change
- fixing a UX problem or
- hacking/overriding the core because an api doesn't exist (eg. toolbar fixes)
Because what I think is happening instead is we are getting distributions instead which are solving UX problems in ways that core plone would benefit from but can't because they have been implemented as one big take it or leave it package. They don't follow the pick-n-mix plugin philosophy that has made plone great. CastleCMS, Quieve, Cynin etc.
Why? because core plone is failing to create an environment that lets them customise plone via a plugin cheaply enough, or changing the core to make the same UX fixes creates too many community arguments
The end result is a fragmentation of effort and community.
Quieve and CastleCMS. Take your learnings (based on actual user experience) and customisations and bring that back to the core... or at least plugins... pretty please? People listen to actual user experience. I hope.