when my brother-in-law decided to get into photography as a hobby, we had a long discussion of whether or not he should go Sony or Canon. as a computer science guy, he was initially enamored of Sony as a company really pushing the technical envelope. browsing some forums, he deduced that Canon was imminently in fear of death due to not shoving all the best technical goodies into the 5D Mark III and this further convinced him to go Sony.
so I discussed the concept that the OP has pointed out, that when you buy an interchangeable lens camera, you are buying into a system. Sony doesn't come close to matching Canon's system. it's clear that Sony wins if you want a mirrorless body, or if you want lots and lots of bells and whistles. to me though, that's pretty much where the Sony wins stop.
lens selection: only Nikon can even come close to Canon. Sony is decades away from having the comprehensive lens portfolio that Canon provides to EF-mount users. and with lens rentals becoming a thing, you really do have affordable access to every single one of those EF-mount lenses.
lens cost: Sony lenses are generally more expensive than their equivalent performing Canon counterparts.
customer support: even for non-pros, we sometimes break things. I busted a 16-35mm f/2.8 L lens horribly, it got shipped out to Canon, fixed, sent back like-new, and has kept on working for four years now. cost me a little north of $200 at the time, and only took a week between sending it out in a box and getting it back in my hand. not using CPS, just support for regular joes.
ubiquity: in a similar vein, if you were to lose/drop/destroy a lens while traveling somewhere else in the world during a long trip, I'm pretty sure the first camera store you waltz into will have something that can mount to your EF or EF-S camera. not so confident about Sony.
confidence in system: Sony has been all over the map with their camera offerings. they're doing what we refer to in the corporate world as "throw it against the wall and see what sticks". you've got A-mount, you've got A-mount DT, you've got E-mount, you've got E-mount FE... now it's NEX, now it's just Alpha, now it's DSLRs, now it's pellicle mirror DSLRs, now it's mirrorless. Sony is a massive corporation that could easily write off their entire camera division and just dedicate themselves to sensors, or rededicate themselves to cell phones, or pull out altogether and focus on any of the thousand other things that Sony does (basically, pull a Samsung). Canon's got other divisions but their camera unit is a fundamental part of their corporate identity. where some people see lack of innovation and boring products when they view the Canon M-line and unwillingness to go crazy with EF-S and EF DSLRs, I actually see a very reassuring constancy in product lineup that makes me confident in continuing to buy their products.
the stuff you don't see: read Roger Cicala's lens teardowns and look at how Canon is paying attention to designing its circuit boards and barrels for robustness and ease of service. also read his experiences with different companies from the perspective of a vendor. check out the Canon CPS room at the Super Bowl or the Olympics. this is why Canon is the apex predator in the photo world. people on forums always talk about how "it's so simple, why can't Canon see this one thing" when in reality, it's Canon that sees millions of tiny details that you, the casual shooter and forum browser, are missing. and it's those things that actually make the biggest impact on the bottom line.
the doom-and-gloom is overhyped: I laughed when my brother-in-law told me the part about how if Canon doesn't "fix" the 5D Mark III with the 5D Mark IV, it's doomed. I told him that everyone on forums said the same thing about ... pretty much every other Canon DSLR ever. I've been around long enough to remember the complaints about the 5D Mark II, the 50D, the 7D, the 1D Mark IV, the 1DX, the 7D Mark II, the 6D... and yet each of these products releases, they sell fantastically, and you walk around and see them in the hands of tons of people who shoot seriously and casually. I told him to actually keep track as we were on vacation and see how many people were shooting with Canon DSLRs versus Fuji, Olympus, Sony, Panasonic, and Nikon bodies. not just draw a data set from one anecdote, or one day, or your impressions from one moment, but very seriously keep track of every camera body you see. we saw a TON of canons.
and finally, word of mouth: when your product occupies a large part of the market, it's easy to find people with lenses that are compatible with your camera. while I'm not a professional, I have photographed for half a dozen weddings as the primary photographer. I've been able to swap lenses with the video guy because, hey, we're both shooting Canon. I've been able to swap lenses with coworkers for fun because, hey, we both shoot Canon. nowadays my brother-in-law swaps lenses with me because, hey, he wound up buying a 6D and loving it.
props to the OP for starting this thread. I don't think it will be read or appreciated that much by the folks who like to come here and whine about Canon, but it needs to be pointed out time and again to new folks who haven't had an opportunity yet to see or understand the whole package that Canon offers.
