Lack of GPUs prolongs profitability for those who already have them, or manage to get some.
However, do keep in mind that NVidia makes GPUs too, and they are very profitable on most coins and can often beat AMD on hash/watt even where they lose on hash/$ (mostly ETH ETC and such, and the Cryptonight coins like XMR).
For ZEC (which isn't the most profitable NVidia coin by far but is probably has the second most GPUs mining on it) NVidia is quite close on hash/$ (and might even WIN on some comparisons) WITHOUT FACTORING IN the current AMD "gouge" pricing.
Funny part - folks have been pushing the Skein coins a lot, but I actually ended up making more on something else (that I'm NOT going to mention here, as it's fairly small and I don't want the profitability of it to get overwhelmed by a bunch of newbies or even one BIG farm jumping all over it).
Mentioning it or not, it doesn't really matter in the end because it eventually evens out with other coins profitability, lots of people know about whattomine, and also people can check what's hot on trading volume on any exchange especially poloniex which is booming hard... probably you meant SIA,(but it doesn't matter)
SIA went up by a lot last couple of weeks and I jumped on the hype train just today, dual mining eth has finally become more profitable for me...
But Sia is just one of many, generally every altcoin has been pumped since last ~2 months...
That is right. As long as the ETH and ZEC is still mineable, the gpu mining will last.