Given the ammunition JP has and Cobden can only ride one horse per race, why wouldn’t he continue riding especially when JP’s first string doesn’t always win a lot of the time.
Yes he will not be riding the marquee horses (unless JP is double handed in races or has entries at multiple meetings or Cobden is injured and/or suspended) but he would still be riding a lot better quality horses than if he went out on his own.
Given the truly professional and classy way he dealt with losing the job and that he continued riding winners as the job came to an end, JP would be crazy not to use him as a back-up whenever/wherever possible.
No reason for him to retire unless his heart is now not really in it and if that is the case you do just walk away….pretty sure JP would have a job for him somewhere in his organisation, hell he would be a far better addition in the role that that misery chops AP currently does.