As the lines between FF and crop continue to blur - and we are seeing very high performance crop bodies ....
Assuming you're referring to Canon crop cameras I think your premise is false - unless by "performance" you're excluding what ultimately counts, image quality. Over the past few generations of Canon crop bodies there has been negligible improvement in sensor performance - the images taken with a 70D don't look much different from images taken with a 7D, 60D, or the last few years' worth of Rebels or EOSM, even if it's now easier to reach that result thanks to better AF etc. The gap in price between crop and FF may be narrowing as the price of the 6D continues to fall, but if anything the gap in image quality between crop and FF has been widening: cf 5DII vs 7D/60D/Rebel and 6D vs 70D/EOSM - the crops have stayed much the same, but the 6D is noticeably better in image quality than the 5DII, which in turn is still considerably better than any of the crops. (And even when/if Canon does start to introduce crop sensors with significantly improved image quality - 7D2? - the various physical traits referred to by the short-hand phrases "crop factor" and "full frame advantage" will remain.)
Is this really true that FF IQ is so significantly superior to crop ? I think the market is moving rapidly to smaller bodies and smaller high quality lenses. Sure there are certain aspects of FF especially in the area of DOF that are not achievable with a crop, but ultimately the prosumer like myself, will not justify the cost of FF bodies and lenses. I still think there is a new much bigger prosumer market - that will go for high quality lenses for crops.
It depends what you mean by "significantly" - the differences vary with what you photograph, and in what sorts of light, and how noticeable those differences are varies with how you view the results (monitor size/quality, print size etc.) - and what's significant to me might be trivial to you, and vice versa. I was responding specifically to your comment that the gap between FF and crop is narrowing. At least until now, with Canon the image quality gap has been widening even though the price gap has been narrowing. If there's a reason for Canon to make new crop-only lenses, it's not because their crop sensors have been improving. (It's true that there have been improvements in the image quality of crop sensors in other companies - esp. from Fuji and in the better Micro 4/3 cameras, the latter also being first rate in other ways - but Canon doesn't seem much interested so far; maybe the 7D2 will change that, but that will not likely be a bargain either way.)
As for the cost of going FF, well yes, it could be exorbitant. But if you don't need the fancy AF of the 5DIII/1Dx, there's the 6D, whose image quality is excellent and whose price, if you catch the right sale, isn't that much higher than a 70D. And FF lenses don't have to be very expensive: if Canon were to issue some crop primes, do you think they would cost less than the 24/28/35 IS series, the 40mm, the 50mm 1.8/1.4, the 85mm 1.8, the 100 f2 or even the 100L or 135L? I doubt it, and while there may be some room for improvement in some of the older primes, they will still provide better images on FF than they do on crop (and if all you've been using on crop are "consumer" zooms, they will provide much better images on crop too). If you want a long zoom, the 70-200 f/4 L IS and 70-300L may be more expensive than comparable length crop zooms, but I doubt "high quality" zooms for crop would cost much less, if at all (check out the prices of the higher end Pentax zooms - not that there are many of them, and not that they're mechanically anywhere near as good as the two Ls I mentioned).
By the way, is it really clear that "the market" is rapidly moving towards smaller bodies and lenses? Some companies may be hoping that it is, and that supply will create demand, but so far the demand hasn't kept up outside Japan and neighboring countries even if the products "deserve" it. (This is a mere anecdote, but when I'm wandering around with my Olympus OMD, even in locations laden with tourists like Independence Mall in Philadelphia or Central Park/Times Sq in NY I can't help noticing that no-one else seems to have such a thing and am sometimes asked whether I'm using a film camera - people seem to have no idea what M43 is or even that Olympus still exists....) The new Sony FF mirrorless may make a difference, but since they're FF there's a limit to how small the lenses will be (the few they've made for them so far certainly aren't small).