Since 2005 I have owned a bunch of Canon DSLRs and have several EF and EF-S lenses, including some L glass.
Earlier this year I bought my first Canon mirrorless early this year - and now have 5 EF-M lenses. (Most bought 2nd hand)
They complement each other.
1) My DSLRs are used when I require fast, responsive photography, and/or with an ergonomically satisfying experience. My passion is nature photography (landscape, macro and wildlife) - and when I am out photographing these, my 80D and 7D do the job. I also photograph events (camps, birthday celebrations, special occasions, etc) using my DSLRs and EF / EF-S lenses.
2) I use my EOS M5 when I require a more compact, yet still capable photography system (e.g. casual hiking - to keep the size and weight down, or when out with my young daughter and already have a day bag quite full with food, clothing, etc).
The M5 in particular does a great job with its DPAF. It's relatively snappy (turning off post photo review gives a much speedier experience). The Samyang/Rokinon 12mm f/2 as an UWA and Canon EF-M 22mm f/2 work well as fast primes. Depending what I'm photographing, I will pull out my 15-45mm (sharp and wide end suitable for landscapes) - or the 18-150mm as an all round / travel lens (great IQ and very useful range). My M10 matched with the 22mm f/2 is particularly small yet still provides decent images and useful video.
Depending on the range, features and price that Canon is going to bring to the table with future R mount FF mirrorless cameras and lenses, I may be tempted to buy into that. I had not felt the need to buy FF DSLR till this point (because the size / weight of a decent FF DSLR with EF lenses offset what I could get from APS-C DSLRs, particularly with my EF-S lenses). If you can't get a great landscape photo with a APS-C DSLR and say the 10-18mm IS lens, or capture action with a telezoom like my 70-300mm (which provides fantastic reach on a APS-C) - then upgrading to a FF will likely not be your best next move.
If you can get good images from Canon's current APS-C DSLR, then you will most likely also be able to get close to matching that with the EOS M range of bodies and languages (in a smaller, lighter package, all bought with a decent price range).
So for the above reasons, I am keeping a ball in each court: DSLR, EOS M and EOS R. They definitely do complement each other. Horses for courses!
PJ