It’s a completely valid reason, but All the talk about platform lock in, independent nodes and relay and whatnot is just to make you feel better (I listened to some talks and podcasts but realized that it’s all just window dressing, you can be practically deplatformed at any time, so I’m hazy on the details).
Credible exit matters too. The idea that all of this data is signed, public, and verifiable by anyone helps ensure that the provider is accountable to the community. A provider might decide against enshittifying a product if they're worried about a competitor doing better and taking their place.
Yes, an atproto lexicon will tend to be dominated by a small number of nodes. It's not the perfect distributed system some would prefer, but it is a real improvement on the completely centralized models seen everywhere today.