I understand why it wasn't possible to expand the platforms to cope with the new Thameslink trains. But it remained (remains, potentially) incredibly useful for Underground access.
If you're going to or from that part of the Kings Cross area (which is my case), it's far and away the easiest / quickest / least congested route to the Vic Line; and before the reshuffling of interchange tunnels at KX Underground, it was for the Picc too. Pre-pandemic, it had quite a bit of use, in the rush hours especially.
My understanding was that it wasn't only kept open because locals wanted it (which is itself a good reason!), but also because it provided an emergency exit from much of the Underground complex in a situation where one of the main exit areas was blocked. And if it had to be kept operational anyway, then why not let people use it routinely?
The problem, in terms of its emergency use especially, was that the "up" escalator from the corridors up to street level was out of action for over a year (pre-pandemic), with repeated promises as to when the repair would be completed not being kept. I was told that the escalator infrastructure was NR's not TfL's, hence the lack of urgency - though I'm not clear whether that's true. (I believe the Pentonville Road station overall was NR's responsibility when Thameslink stopped there.)