Err... as nobody seems to want to answer your 
questions, here is my opinion...

Are cels still worth to be collected ? That depend 
what are your personal reasons for collecting 
them : Most people don't collect things because of 
the hypotetical value they could get by reselling 
them. Also, there isn't much point in collecting 
things that are common and easy to get.
As there will be less new cels on the market, 
older ones will become rarer. And so they will be 
more collectable.

What is the future of cels in the anime industry ? 
I fear there isn't much of a future for them : 
nearly all new anime will be done without cels.

Probably we will see more limited edition cels 
produced but that may not interest people : They 
are less interesting to collectors as they aren't 
used for the production of the anime. And they 
cost more to produce (and so have a higher price,
whatever the collector value) than production cels
which are just a by-produce of the anime itself.

Will there be less people collection cels ? Yes,
it's quite possible : People that aren't yet
collecting and that are interested by new anime 
won't find cels so they won't "enter" into this 
hobby. But usualy there isn't many people 
interested in cels from new anime anyway and they 
want older, rare, more valuable ones. So...

Cheers,
Yann Stettler