Originally Posted by Povl Carstensen
Well I am 52, run and play tennis without shoes, no problems with my knees.
As you have said ad nauseam. Everyone is different, with regards to both their joints and the level at which they play tennis. Being stronger at the former and weaker at the latter will aid knee health.
To the OP, just buy a pair of top of the line warrantied shoes on clearance for $80. That works out as just $40 per pair and the shoes could last you a total of one whole year. Even if they only last less than that, it's still a small price to pay in my opinion.