It's not just spectrum, though. The core network is totally different than 5G SA or whatever 6G has. Maintaining all of that will eventually be considered not worth the trouble, and it'll be decommissioned. If anything decommissioning will be faster for LTE because unlike 2G/3G (which were needed because VoLTE support was flakely and had device whitelists) there isn't much point in keeping LTE around for legacy reasons.
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.
