I'm really not one to ask about what choir library to buy in that my experience, historically, is mostly buying the wrong ones.
But I think the interesting question here is how the notion of a 'mainstream' choir library has been changing, certainly with EWC, but beginning - at least in my understanding - with Dominus.
Before Dominus I think it was fair to say that a 'mainstream' choir was more or less an epic, or maybe symphonic, choir. At least in the sample library world this was true, because of course in the world of really singers actually singing in actual real choirs the epic was never so dominant as to qualify as 'mainstream'
And you see this addressed both subtly and not so subtly in the interview with Eric - I really like his quote about a choir having the range of a symphony. Which at first sounds like a Blakean 'world in a grain of sand' sentiment. And to some people it surely is - EWC has all these very finely crafted detailed textures that go to incredible depth. From the perspective of the expressive dimensions of the 'conventional' epic choir, it might very well seem like the diminishing returns of zooming in to incredibly fine detail - like counting grains of sand on the head of a pin. A variant of the 'all you sul tastos sound the same to me' response to Tundra.
... except that maybe the world of choirs - if we step away from the dominance of the epic choir in the sample library world - maybe the EWC level of expressive detail was never a grain of sand, but always a vast sea of enormous, symphonic, expressive dimensions. Which is something that I began to feel when Dominus came out - igniting an angst for a different type of choir. I have Venus, and Mercury, which for all their extreme loveliness, never seem to do quite what I want them to. Dominus felt very different.
And then ... Time Marco hit, and its choirs were both a huge surprise and an instant revelation. It was suddenly clear just the kind of dimensions choirs could reach towards - even if there's really only one or two patches among the TM choirs that align with what I'm looking for, they really are just that stunning in how they suggest new possibilities of the choir. (The rest of the patches are fun but a bit ... well the "zzz"s and "nana"s would make a great muppet choir if you could only throw in a 'phen-om-a-na' patch - which now that I think of it,I really want).
Of course the TM choirs are also very limited. I attempted to mock of Eric's "Lux Aurumque" with TM ... which I can share if anyone's morbidly curious ... but the bottom line that while some moments within individual chords are indescribably gorgeous and suggestive of vast possibilities ahead ... you simply cannot and should not attempt Eric Whitacre mockups with TM.
One of the first things to go through my head wth when I first played a TM choir patch was ... 'what if spitfire were to do an entire library along the lines of 'choral swarm'! .... and then EWC hit (though to my deep laminations, it will probably next year before I reasonably (or unreasonably) justify another large sample library. Sigh.)
But to (sort of, if actually, not quite) answer your question ... sure, why not! I myself am totally comfortable with letting the Dominus, TM choirs, and EWC entirely redefine what a 'mainstream' choral library is - let the supposed grain of sand become the new universe. And within our brave new choral universe, relegate all the formerly mainstream epic libraries to respected, but actually, when you think of it, quite small places on the periphery.
Needless to say, you mileage may vary.