You lose a lot more than just message bubble colors when you switch away from iMessage. Multimedia quality takes a nosedive to 2006, reactions don't work, you lose encryption (SMS/MMS is not encrypted at all), and you miss out on all the other little things that make people like iMessage.
Of course this isn't really Apple's fault, we can blame the cell carriers for being categorically uninterested in modernizing SMS or making it secure.
Is multimedia quality between Android phones using MMS much better than iPhone-without-iMessage? I thought it's the same, which isn't lock-in, that's just iMessage being a superior service to MMS, and in fact the iPhone is going out of its way to not lock you in and allow MMS.
On the other hand, if the iPhone is intentionally crippling MMS, that's pretty slimy lock-in.
SMS is just as bad on Android phones as it is on iPhones. The problem is it's an ancient crappy standard and carriers are uninterested in fixing it. That is why Apple created iMessage.
Apple does nothing to cripple SMS on iPhones. The only thing Apple is guilty of is not opening up iMessage to non-Apple devices. With Apple Music and TV+ making their way to other ecosystems, I'm hopeful this might change.
Of course this isn't really Apple's fault, we can blame the cell carriers for being categorically uninterested in modernizing SMS or making it secure.