One of the oft overlooked issues in the APS-C to 35mm format discussion, when considering 35mm format lenses to cover the same focal range as the 10-18, 18-55, and 55-250mm STMs is price. All three lenses give very good optical performance, generally quite a bit better then an almost equivalent focal length range in the consumer EF mount lenses. I had a quick look at my local dealer, the three EF-s STM lenses come in at a total of £687, including all our taxes. I beleive that the 24-105 STM is in the same IQ class as the 18-55 STM, and although a much longer FL range at only £379 it is much cheaper than the equivalent 24-70, which being an L pushes it up to £819. To get any Canon EF mount lens close to the width of the 10-18mm you will have to at the minimum go to the 17-40L, the closer matching 16-35mm being another £150 odd. Then you hit the real snag, with the telephoto, since none of the EF mount consumer lenses get anywhere near long enough to match that 250mm on the APS-C sensor. Canon simply don't have a consumer grade xxx-400mm zoom, and I'm not sure if the new MK II 70-300 IS USM will come anywhere close to the EF-s zoom optically, and even if it did it would still be 100mm or 25% too short. So ouch it's the 100-400 which in version 2 guise, since we are comparing new lenses after all is an eye watering £1789, or £1520 more than the 55-250. That makes a grand total of £2887 to match the coverage across the board of those three EF-s STM lenses. That is an additional £2200 to match the focal length range available in the smaller format. Although Canon have had a relativly basic introductory 35mm format DSLR body on sale now for around four years IIRC in the 6D, they simply don't have a corresponding range of optically good consumer EF glass. If Canon were to have a matching range of EF lenses to those for the EF-s mount, at a similar price differential as between the 80D, and the 6D, those consumer EF lenses should be around £1000 in total, or if you make the comparison from the 750D, then you are looking at a total of about £1800, or two thirds of the actual price difference.
Yes those EF lenses are of a much higher level of build quality etc, and yes from that point of view it's true those lenses are probably as good as it gets from Canon. Well the 100-400 certainly is, the 17-40 is still an L series lens, and the only "consumer" grade lens in the EF list is the 24-105 STM. A consumer grade 17-40 is badly needed, now that APS-C has a really good option in a similar FoV range, and the consumer telephoto zooms are simply dire, none of consumer 70-300 really make the grade compared to the 55-250, or you would see them recommended for use with the APS-c bodies too, since for a lot of uses that extra 50mm would be very very useful.