Depends upon your needs. Whilst I think it's a great compact option on full frame, for me it's too long and too slow for a true fast-50 equivalent on APS-C. The Sigma 30mm f/1.4 is the closest you'll get in EF mount, as Canon doesn't think it's worth making one.
Now repeat the exercise for a fast 35mm equivalent? That would be either the 24mm f/2.8 IS (a bit slow) or the 24mm f/1.4L (heavy and expensive).
How about a fast 24mm equivalent lens: Canon 14mm f/2.8L, Samyang 16mm f/2 or Zeiss ZE 15mm Distagon? All have pretty obvious drawbacks!
70-200mm f/2.8? -Third party again (Sigma 50-150 f/2.8 or Tokina 50-135 f/2.8 ) are you're only options.
Is it any wonder that plenty of photography enthusiasts are looking at the increasing capable mirrorless options? Canon are at risk of losing the middle ground between the "Soccer Moms" (a lovely American marketing term!) and professionals. If they want to secure this ground, it's lenses not bodies that they need to address.
When I used crop cameras, I had the 10-22, 17-55 and supplemented it with a 24L and 35L. The Sigma 30 f/1.4 is not nearly in the same league as the Sigma 35 f/1.4, and at 500 the Sigma 30 is also pricy for what it offers. The 35 f/2 IS is a nice lens and could be had for less than 500 during one of the Canon refurb store's sales. Smaller format cameras are at a disadvantage when light levels are low, so faster glass is more necessary and flash will be used more.
If I were looking to outfit a crop kit from scratch, I'd seriously look at Sigma's 18-35 f/1.8. With that lens, you wouldn't need 24 or 35mm primes. Add a Tokina 11-16, and that'll satisfy most for the wide and mid range. I'd then supplement it with a 50mm prime and a 70-200. I like the f/2.8, and wouldn't give it up. I used it for sports, events, etc. I think 3rd party offerings strengthen the APS-C offerings; not everything needs to be Canon.
The 6D can be purchased new for ~1500, which isn't that much more than the X-T1, and I'm sure the price of FF will continue to drop, which will threaten premium APS-C systems like Fuji. The Fuji system is more compact, but the lens family is much smaller than Canon + 3rd parties. Plus, the Fuji lenses are expensive too. I like the idea of a Fuji system, but the lens/flash options are too limiting. When I need to use something compact, then I use the M. It's not as nice or capable as the Fuji, but then it had cost a lot less and I can still use all my lenses on it if I had to.
The original Sigma 30mm f/1.4 (non-"Art" and optically identical) version is on run-out special, here in the UK at least, for £279. The "Art" version is £369, which is still cheaper than the EF 35mm f/2 IS at £459 (although I would probably recommend the Canon lens as worth the extra money). None of these is really at the same price point as the Nikon 35mm f/1.8 DX lens, which is why I think that there is still a gap in the market for a Canon equivalent.
I sort of agree when it comes to the Sigma 18-35mm f/1.8, but it is very heavy and quite expensive -£629 is quite a put off to the owner of an entry level camera that might have been purchased for half that amount. Whilst a set of fast primes may end up costing and weighing a similar amount, they have the advantage of being purchased over a period of time and not needing to be mounted on the camera at the same time. The problem with Canon's current strategy is that you're forced to use full frame ultra-wides with all the attendant size, cost and speed disadvantages to fill in for non-existent dedicated APS-C wide angles.
I wouldn't like to comment on the production costs of a 6D versus a X-T1, but one could turn your argument around and point out that the X-T1 currently manages to hold 75% of the 6D's price despite being only APS-C. My guess would be that whatever cost savings can be made on a full frame camera can also be applied to an APS-C camera. Besides, the real cost of jumping up a format size can often be measured in lenses rather than just the body.
I know that one runs into thorny ground with the whole lens equivalence question, but I think that if absolute depth of field and/or low light performance are critical to your style of photography, then you're probably one of the people for whom full-frame-35mm will always make sense. If you're prepared to accept some compromises, sub-frame can make sense; it's just a question of which brand offers the most for the least...