I think EE does have an interest in getting as many people using 4G Calling as possible. Maybe not quite so much about people getting access to 800, although from a customer service point of view and for producing facts and figures, that would be good too.
Not every old phone will be done, and there are issues with some manufacturers, so I am not kidding myself into thinking there's going to be some massive action to enable every 4G phone ever made - but where a manufacturer has made VoLTE work on EE's network, I hope older phones that can use the same settings will.
People will have other incentives to upgrade their old phone than it suddenly offering 4G Calling - a feature I doubt people would be that bothered about, perhaps not fully appreciating what it actually means. Especially to those who think 4G carries voice already anyway (and why not think that?).