I think it's impossible to get a good bead on how 
many people drop out.  I know 2 friends of mine 
from the old days (or at least, MY old days, when 
I started out around 3 years ago) don't have a 
forum presence anymore, and I'm sure there are 
plenty of others who I never noticed who've 
disappeared in that span of time.  But then there 
are people who have been collecting for years and 
who still do so actively.  It's impossible to say 
for sure.  

If I had to tender a guess as to why people 
leave, I'd say declining interest in anime in 
general, coupled with newer hobbies that could 
absorb money to better effect >:D.  I seem to 
recall Hachi-san diverting his efforts from cels 
to guitars--wonder how that worked out for him, 
really.  I miss that kid.

As for what would make me quit, I'm almost 
there.  I need 2 more cels to make me quit for 
good--a better shot of Lt. Andressen from Tylor, 
and a genki shot of Rei-kun from Kodomo no 
Omacha.  If I tracked those 2 down, I'd have 
almost everything I'd ever set out to find, and 
given the rarity of the 2 other things on my 
list, I'd say I'd have everything I could 
conceivably find.  That's not to say I won't buy 
other cels between now and the time I find those 
2 cels; it's just, once I have those, that'll be 
enough for me.  I counted my collection 
yesterday: I've got around 60 cels, or will when 
my outstanding lovelies return to me, and I'll 
have spent in the neighborhood of $2000 in nearly 
3 years.  I'd like to keep it that way ^-~.

God, I wish people would come back from AX 
already...without them, I have too much license 
to ramble.