I usually have 2 pairs which I rotate. When I buy a new pair, i retire the pair which is most worn out (tread or upper, i like my shoes to fit like gloves).
However, i don't slide on hardcourts, only on clay, but I'm 31 and want to keep playing for a couple of decades at least. Have already injured my ankle (running injury, not tennis or squash. Go figure).
Disclaimer: I'm NOT a coach...
Real tennis: https://www.youtube.com/watch?v=yDqnkLJ9BtM