Keep my tubes??

So, I decided to cash out of my tubed-based system about a year and a half ago. I intended to get back in when I could. At this point, I'm not sure when that's going to happen. In my day dreams, I'm even considering going solid state amplification.

My question is, should I hold on to my tube collection or, cash that in too?

My instincts tell me that these are old stock tubes are a limited resource and the supply dwindles by the day (at some point, there will be no more right??)

Since I've been out of the hobby for awhile, I'm trying to understand the other side; the demand side.

Should I hold or sell?