I'm wondering if anyone else has this problem.
 For years and years, I've basically collected anything
 non-Wintel old-ish (older than ten years) computer-related stuff.
 I've got quite a few systems which aren't really "rare" like Commodore
 64, Radio Shack Cocos & MC-10s, TI-99s, etc. etc. 
I sometimes (mentally) kick myself because I didn't buy up 'common' home
computers about 20 years ago (things like ZX80s, TI99/4a's, etc) and now
I don't have them in the collection. On the other hand, I am not really a
collector, more a hacker, so I am glad I bought the obscure stuff (P850,
PDP's, Nanocomp, etc) which is probably more interesting to work on.
 I've also been amassing a collection of DEC PDP and VAX "stuff" which
 I work on, use, and enjoy on a regular basis.
 My basement is a mess and I'm starting to think about scaling back by
 selling or giving away some of the non-DEC common stuff.
 (I regularily see Commodore 64s on eBay for $20)
 Is this specializing the right approach?  Am I likely going to be kicking
 myself in fifteen years because I *had* a working Radio Shaft Colour
 Computer and I eBay-ed it for $10? 
Depends on what you're really interested in. If you like minicomputers,
and like them becuase they're multi-user machines, or because they've got
processors built from TTL that you can get inside with a logic analyser,
or... then you might as well not bother with the home micros. If you want
to have a more historically-based collection then you probably should
keep the micros too.
  Also in my mind is the possible demise of analog TV
and it may not be
 possible to find a TV with a composite-input to plug my C64 into in
 fifteen years if everything (including broadcast) is digital 
Well, I would always want to keep at least one composite video monitor
around, it's useful for all sorts of purposes. And of course good
monitors can be repaired.
However, I suspect somebody will still be selling composite-input boxes
for whatever display is current in 20 years time. There's so much stuff
around with composite outputs that people will still want to use.
-tony