Sorry, what I meant to ask is if such a mutation could go beyond the chronically infected cat and replace other FIP viruses.

I never knew that! I just read that it can turn fatal due to a mutation, but do these spread in cats like COVID variants? Omicron and Delta already demonstrated that outcompeted variants can come back far worse.

They also continue to exist in chronically infected people, mutating over the course of infections that could last years(or decades?)

These are 3 Delta variants detected this January, one of which was up to 90 mutations(and probably still mutating):
https://twitter.com/LongDesertTrain/status/1624464486596849670

Non-Omicron variants like Delta are only extinct in the sense that they are no longer widespread in the general population. Even so, they continue to mutate in chronically infected people for years, resulting in variants that make even Omicron look "pedestrian." https://www.nature.com/articles/d41586-022-02996-y

Here are three Delta variants detected this January, one of which got up to over 90 mutations: https://twitter.com/LongDesertTrain/status/1624464486596849670

This might be how Omicron started and how the next big variant emerges.