If 90% of miners stopped working (which is completely unrealistic) then I am many many more would start mining I'm sure.
I'll put 50 PC on the job right away. There are some issues with bitcoin but that there are not enough miners is not and will never be one of them.
There isn't much incentive into attracting people to mine if the difficulty remains the same and the profit would still be low. It can be a problem since the difficulty wouldn't be reasonable till a few difficulty adjustment later.
The fewers miners the better the chances to get alloted btcs for the rest, right?
So as (or if) miners get fewer, more and more people would start mining.
in other words, miners cannot be fewer, for every miners that leave the field another one will replace it, like it was said, bitcoin network is self-sustaining
this will work as long as the bitcoin price is good and there is enough demand, if the demand start decrease too much, than miner will diminish slowly until we have a problem regarding security and double spend, at that point i'm not sure if bitcoin can recover