I agree, but also disagree.
There may be some Canon users who flirted with Sony who will switch back to Canon eventually. Even going back a few years I was chatting with one pro who switched to Sony. He was wildly singing Sony's praises. I saw him at another event six months later and noticed he was back to using Canon. He said he wanted to like the Sony but just found himself disappointed and ending up switching back. (I know, a sample of one). But, I agree that most people who either started with Sony or switched to Sony will stay with Sony.
I don't think Canon expects to capture those customers. Instead, I think they are looking to hold on to their base and maybe pick up a few new users. That's all they have to do. They really don't need to convince Sony customers to switch, as they already have half the market.
You are correct in that the bulk of the market and bulk of the sales are at the lower level. But the enthusiast market is the high margin market and the least likely to abandon cameras for cell phones. Canon is highly interested in retaining that market and is well-positioned to do so given their dominance.
It's hard to tell the overall pictures with anecdote stories. I know of 1 pro who switch back to Canon but majority of people stay and more are are slowly switching over or to another brand (4/3, Panasonic, Fuji, etc). His reason for switching back is to keep consistent editing style for his studio and skin tone. Some are using dual system even since adapter works really well with MC-11. Sony know how to use social media influencers to capture the younger crowd.
There are going to be people switching back and forth but what is the overall net flow?
Here is the headline recently
Sony 2018 financial results released: Imaging Business had +2% in sales and +12% in profit compared to 2017
Canon: In the first quarter, sales of interchangeable-lens cameras were down 19%
My point isn't to say Canon isn't dominant, they will be #1 sale in a shrinking market, but they aren't going be as dominant with less sales and less overall percentage as before when there are only 2 major players and alot of less competitions from different formats when the market isn't saturate.
Smart phones, DJI, Go Pro, and all the emerging products will steal sales away from low price high volume market Canon used to owned.
In the enthusiast market where like you said, "the high margin market and the least likely to abandon camera for smart phones", you have Nikon, Canon, Sony, Panasonic, Fuji (APS-C, affordable medium FF) compared to the yesteryear of fewer competitions.
You have emergent of Tamron and Sigma making quality affordable lenses that eat into Canon overall lenses profits. You have photographers who still don't want to upgrade their DSLR to FF mirrorless because they don't see the value of upgrading since their gears work fine, financial reason or technical issues (EVF).
Photography market is a very saturated market with alot of competitions just like the PC industry and that's why Canon is diversify their porfolio in other fields beside camera market. I really hope the competitions will make Canon be aggressive with feature sets and pricing instead of finally offering features that they are the last to withhold (IBIS, etc).