Disclaimer: I don’t have anything against the relicensing to LGPL. I think it’s your right and I root for you.
That said, correct me if I’m wrong, but, as far as I understand, what Redis Ltd did, they could do regardless of the license. Copyleft wouldn’t have stopped them, given the CLA.
Moreover I wouldn’t call that exploitation. To people outside of Redis Ltd who don’t want to be Redis Ltd customers this move is indistinguishable from them just closing down business and stopping development of Redis. Would that be exploitation? Are they obliged to provide free work on Redis indefinitely? They can’t retroactively change the licence of previous versions of Redis, so they can’t actually take anything away. The existence of the 2 forks is proof of that.