I always think with double glazed windows, it's how often they've been open and closed that can lead to deterioration.
Ours now are twenty years old. With the bays we've only the centre one of the bottom five windows that does open and we rarely open them, just the single top light above . Over the years two bottom windows and one top light in one of the bays have "blown" and we had to have those replaced.
I replaced all the seals a few years ago. It's not expensive, you can buy it by the metre and it's dead easy to change. But you need to take a small piece to a window manufacturing company as there's all sorts of profiles.
I think ours "will see us out."
Old D/G windows are easy to take out if they are just flat pane windows, but you need to check that the course of bricks over the top aren't just being kept in place by the window frame. To be on the safe side. I'd use a length of 3" X 2" with a bit of plywood under it on the window sill and another on the top with the lot jammed up against the centre of the top of the aperture, before attempting to change a window.
It's old-fashioned wooden window frames which were the problem, particularly those fitted when houses were built, as the bottom and sides of the frames extended a few inches at all four corners. When the double glazers removed them, they could have disturbed the brickwork, but should have made the corners good before fitting the new windows. These are just screwed in.
