The only way I could ever get any of My 5D mark-iii's to focus in low light was by sticking a Canon flash on top and using the AF assist light, once that was on it did a good job but far from perfect. Take that off and the Fuji's shamed the Canon in low light AF accuracy. Mirrorless cameras can not use IR AF assist [Shame that] & there is no way I ever going to turn on the AF assist LED. Shooting with a fast AF lens [LIke the 16-55 f/2.8] and the X-PRO2/X-T2 is more that good enough for my needs even kids junked up on sugar. Even the 'slow' 56mm f/1.2 does a decent job these days, kaizen at work, something I love Fuji for.
It was the consistent accuracy of the AF that drew me into shooting more and more with my little mirrorless crop camera, that and the amazing files that popped out of the camera, JPEG or raw both were very good, I honestly had a very hard guessing what camera took what shot once I graded them up. In processing find the Fuji files have a much better dynamic range than the 5D3 too but that wouldn't be hard. The 5D3 is a wonderful do it all camera that is hard to break and Canon make some truly great lenses to match it, as do Sigma, really nice to shoot with them all. Aaaa memories.
There are pros and cons with all camera systems and I'd love to be able to support more than one, but for the little time I might need a camera to fill a gap where using the Fuji would be to difficult I can always rent for the few days. If you look at the progression of what Fuji have done with their system over the last five years, the X-PRO1 is 5 this year, it's really progressive & I'm looking forward to seeing the continued development that is pushed from the factory. Canon I find are very slow in that regard and they keep good things only for their 'Pro' cameras like the 1Dx. Never offering any firmware upgrade to add newly developed features and improvements. (OK the 5D4 is getting c-log soon but at $99 for the upgrade when the camera should have shipped with it already). This frustrates me as I feel like I'm be ransomed into spending €7k on a 1Dx-ii [probably double then as I need at least two cameras] and having then to carry that bulk about, just to get the features I need if I want the best they have.
You should keep the system that works for you or if you can keep a split that covers you - best of both worlds then. I did the latter for a year until I figured out what I truly needed/wanted: Fuji & happy for that now.
As long as you keep shooting...