Fundamentally, what Bambu are saying is that they have a right to restrict what software accesses their network. The C&D was allegedly sent to stop distribution of software that was written to access their network in an unauthorized fashion (Allegedly according to their ToS).
AGPL covers source code. It does not cover who can access what network with AGPL'ed software.
Thus Bambu - like it or not - have a right to limit what software accesses their cloud. You are still free to do whatever you want with Bambu's AGPL'ed software. But they don't have to let you on their network if they don't want to.
With that out of the way, sending a C&D is a pretty regarded way to accomplish this. The correct way would be to sniff out which clients are using 'real' Bambu Studio and which aren't. However according to Bambu, Pawel specifically modified BambuStudio (ya know, because they haven't violated the AGPL, because he is free to do that) to make it look like Studio.
I can only assume that actually locking down their network for real would require every Bambu printer to have a firmware update that would add some sort of signed encryption to access the cloud features. The C&D appears to be a shitty action prior to a huge undertaking.
I do wonder exactly how secure their super spendy "Enterprise" X1E printer could possibly be given how easily Pawel was able to make a fork work on their cloud.
As to your second paragraph about functionality and theft, 1) I can still print from Bambu's cloud with my Bambu printer so I don't think they've changed any functionality, and I can still use Orca in LAN mode. and 2) designed obsolescence exists.
I disagree with your assertion that because forks were able to access cloud functionality previously, that Bambu must maintain that functionality ad infinitum. My opinion would change if anyone showed me where previously they were promoting how any third party apps could access their cloud.