1. If it will drop only cosmetic thingr - people won't run it.
2. If it will drop non-cosmetic gear - people will feel forced to run through these raids again.
Just find some friends. Everything up to t14 (excluding ofc) can be done in group of up to 3-men (except maybe couple bosses). I don't see problems with raids becoming obsolete and under-mannable as they serve as milestones in constant progress.