I agree it would be easy to get tickets, but if you like your seats and if you intend to go to most if not all games, then why not renew?
Also, they'll probably be a stampede to get STs again by all those who walked away as soon as we went down once we get back into the PL. As a new member I'd lose all my points and be at the back of the queue so may not even get an ST, certainly not one in such a good position.
I haven't decided yet if I'll renew or not if we go down. I will if at least one of my two sons wants to carry on (I pay for both tickets anyway) because I enjoy the day out (not so much this season obvs), but I probably won't if it means going alone.
If I don't renew though, that will be it and I'll walk away from football. Permanently. If I can't support my team of 50+ years through another relegation then I'm done.