Big enough that most carriers can't even launch 5G with its associated core network ("5G standalone"), and had to use the kludge of 5G + 4G core network ("5G non-standalone)
Doesn't sound incompatible. So far these new architectures have just been different ways of describing mostly the same thing. E.g. the NodeB from 3G becomes the eNodeB in 4G, and it's not actually a different thing, it just describes one that has been upgraded with 4G support. In practice the exact same device is both NodeB and eNodeB in a 3G/4G network.