I'd replace them with perch rests, tbh. I think that's the easiest solution to give people somewhere to rest when all seats are full but also encourage people to leave the area free for as long as possible.
By choice I'd put nothing there and keep it for standing space on peak trains where bikes aren't allowed, but perches are better than tip-ups, as the latter are quite desirable for some, wheres while a perch is more desirable than standing it's not more desirable than another seat.
There may even be a lesson there - if people travelling alone do like side-facing seats, fit some elsewhere. That also adds standing space. Might almost be as good as standbacks.
Fundamentally, good train design minimises passenger conflict. There should, so far as possible, never be two valid claims on a space, and if there is signage should clearly state which must take priority (and does in some bike and wheelchair spaces I think, but not every TOC does it right). The French were bang on with their classic sign of "if there is a dispute on the state of the window it is to be closed". OK, I'd prefer it open, but it clearly states what happens in case of a dispute, and that way there can be no valid arguments, just tap the sign, do what it says and get on with your day. I'd have far more of this, e.g. "if reservations are not shown they do not apply; in this case take any seat", "if there is dispute on the position of the window blinds they are to be left open" etc. (Those are my preferences, the other way works too, the key is that the expectation is set and clearly written).
