It was never intended as solo-only content. Ion’s description of delves (well before beta) was that they would be for solo play or small group of 2-3, but accommodate up to 5.
There’s no slippery slope, since they were always intended for both solo and group play. What’s new is that solo players have a path to better gear with Tier 8 delves matching +5 keys at end-of run and +7 keys in the vault.