You already are rewarded for loyalty. If you want to swap Covenants, you can level up multiple characters and swap between them on a per-encounter basis or have one character for M+ and one character for raiding. You are rewarded if you play only one character. Playing multiple characters isn’t a punishment, it’s how you play a video game if you want to min-max.
Folks need to simply accept that their “play one character” mentality is not compatible with min-max gaming mindset. Bring the player not the class was a mistake pure and simple, it bred entitlement and false expectation in players. WoW will never be a game that supports a player being 100% optimal in all content with one character. Warlock is one of the best PvP classes but one of the worst M+ classes. Blizzard won’t be able to resolve stalemates like that without screwing up either PvP or M+ in some way (PvP already has its own weird sub-culture of PvP only abilities and balances making it feel more and more disconnected from the game - war mode is probably one of the few things keeping it rooted in the world).