When I last saw my consultant a while back, he said I'm a few years away from one, my worse knee first. I have RA and joint hypermobility but the main issue in my knees is that my kneecaps don't track properly so are causing damage, like osteoarthritis, and are at risk of dislocation. So it's full on getting physio in to try and get things strengthened up as much as possible before any joint replacement.
As my mobility isn't great anyway and I'm limited exercise wise, it was put that if I put the hard work in now, rehab will be much easier, and that, without it it could (for me) be a nightmare.