Seems that way. I also hooked up my PS2 slim recently to throw in some games and ended up having to switch to my PS3 fat because it was just grinding during the intro video that it was trying to stream from the disc.
Same here. My fat PS2 died a long time ago, and now my PS2 slim is dead. Tried hooking it up over the holidays at my parent's house so my father could play Burnout and GTA, but nothing was reading.
The PS2 slim has two diagonally-opposed black switches that correlate to the opening and closing of the top-loaded disc bay, to force the disc to stop spinning when the bay is opened. There are methods of making your PS2 work by using electrical tape (or solder) on those switches to force the disc to always spin.
Neither the soldering of those two buttons nor cleaning everything in the system worked for me. I watched the optical sensor move and begin reading the disc before saying that it was unable to read it. This happened for all discs.
I'm not sure if the PS2 Slim uses the CMOS battery RAM to sideload games, but it would make sense that something lost a charge somewhere and is the reason why systems are failing despite not being used. The optical board shouldn't fail without being used, and the optical reader itself should only fail to read if it's damaged in some way.
Might be worth a try replacing the CMOS battery inside your PS2 Slims with a regular ol' watch battery. Mine is a few states away, otherwise I'd try it.