I saw someone else pointing out in the thread that fully remote companies would, in time, probably adjust their salaries too. (EDIT: ah, oops… it wasn’t someone else, it was always you!! Sorry!)
As an employee, in the short term, I like to e.g. keep a London salary and save on housing and commute by moving to Manchester. But in a fully remote company there would be no “London” salary or London office at all, so salaries would be likely reflecting a blended national job market.
The transition is certainly awkward for existing companies, though, as nobody wants a salary cut (which by itself could be a good explaination for them wanting to maintain the previous in-office status quo).