I guess, I just don't see the implied option that they could have "stayed anyway." From the way Angel acted, it seems that he was cutting off all contact with AI: living in a new place, probably not having a phone and certainly not a phone number known to Cordelia, Wesley, and Gunn, etc. When someone is cutting himself off that thoroughly, there's probably no chance of reaching through to him anyway.
And this is so even if the AI team could have gotten in touch with Angel through a few mutual acquaintances: probably Lorne, maybe Kate, possibly even Merle. After all, the mutual acquaintances may also not have had any way of initiating contact with Angel, and it's not always a good idea to make or comply with a request like "Hey, can you tell me where to find this guy who's avoiding me?" Telling the AI members where Angel lives would probably be against Lorne's professional ethics; and Kate and especially Merle may not even know how to initiate contact with Angel--all that happens is that he visits them.
So the Fang Gang didn't really have that much of a chance to make contact with Angel, and I don't see that it would have done any good if they had managed to get someone to deliver a message like "We still care about you." (Since there's probably no way to talk to him in person, that kind of message is probably all they could say in a message delivered by mutual acquantainces.) Angel probably knows that they still care, but thinks that it's best for everyone if he never sees them. Like Whirlwind says, their only choice was whether to keep up the good fight without Angel, and that is exactly what they did.