Porlets will likely get replaced by Mosaic and tiles. From a UX point of view it's better because tiles can be used inside content and as a replacement for portlets (ie in certain areas of outer page). Backwards compatibility is always a consideration but given there is almost always a high cost in upgrading a plone site, I don't think it's the highest consideration.
ContentWellPortlets allows that, too: Above and below the content, and between an item's title and the body-part. BTW, do you know if Mosaic and tiles allow per contenttype- and group-assignments as well as portlets do?
Smooth upgrades aren't so expensive, yet migrations are, which I suspect will become necessary here also.
E.g. switching standard content-types from Archetypes to Dexterity works very smoothly, yet when there's more complex custom-types users are forced to rewrite a lot of stuff, if they want to stay up-to-date.
Yupp, but customizing an existing one, requires an override of the viewlet and cannot be done TTW.