It will probably be around, but in a limited way. 
Production art will always be collectible to 
someone, but sketches are an extra step "removed" 
from the on-tv-process, and I'm sure a lot of 
studios will try and go more and more paperless 
as time goes by.

The newer fans probably won't be able to sustain 
the hobby so well. For older shows, you had 
massive fandoms where the fans latched on to a 
particular show and were dedicated enough to buy 
all the anime-related goods they could... cards, 
plushies, toys, cels, etc. 

But few of the newer series have that kind of 
dedicated fanbase and longevity... shows seem to 
go to "flavor of the week" mode, where you sit 
down, watch your 13/26/however many episodes of a 
series over a few days, and then run off to watch 
the next show. Regardless of how fun the show is 
to watch while they're watching it, it's easily 
forgotten because anime is so much more 
mainstream now, compared to five or ten years 
ago. That sort of fan isn't going to be the type 
who will cough up $50, $100, $200+ for a piece of 
plastic (or a piece of paper). There will always 
be people who collect production art for whatever 
reason, but I'm not seeing many of those people 
in the places I hang out. ^_^

No idea what will happen to my cels when I die... 
perhaps my friends will generously offer to take 
in the poor orphans, since my husband has no 
interest in them. ^_~ And as for longest time 
collecting, it's not me... I bought my first cel 
in '98 or so, but didn't start seriously building 
a collection until about two years ago.