I meant viability not in terms of high volume/profit for the makers of slide film. Instead I meant that slide film as a medium is only useful if you can get a range of different films with a choice of contrast and saturation levels. I wouldn't bother with slide film if E100VS (my absolute personal favorite) was the only choice, as certainly not every subject warrants its level of contrast and saturation.
It's not useful, then, if availability of a slew of films defines that. It's pretty much over as a photographic material. That just gets more obvious as E6 lines close and films disappear.