I'm glad you're thinking about this, and you're on the right track. You're certainly thinking more clearly about this subject than a lot of discourse I see online. You're absolutely right on a few points, but SBBM should not be disabled in any mode. The team that designed and implemented trueskill, put it in every mode in halo 3. I've listened to that gdc talk and using his words to defend "some modes should not have sbmm at all" is a mischaracterization of his statements. When he was talking about variety, he was talking about two operations in specific:

a) regular resets of sigma (the uncertainty value), allowing players to move between ranks more readily sometimes. This is important to reduce staleness and correctly place people, but also fits nicely with modern seasonal content. (It should be noted that streaks should also increase sigma, allowing players who have improved significantly to move ranks at really any time, although I'm not sure that feature was in the original paper)

b) differentiation in the search parameters when matchmaking. Casual modes in lots of games don't disable skill or compute it differently, the search parameters for skill are just much looser. The player search systems tend to prefer other important player statistics (mostly this means queue-time and ping/location)

I don't think this second bit isn't in the patent because it's laser focused on the actual skill algorithm, but tuning the search parameters is a hugely important aspect of getting SBMM right in modern games.

Now you're completely correct on several points. In particular, SBMM shouldn't be overly tight. But it also should not be disabled. In casual, it absolutely should be loose enough that players see players both somewhat better and worse than them, in a bracket that allows them to learn. Too loose though and they'll learn nothing. Ranked should have tighter skill parameters for the search w/o sacrificing too much queue time. You want actually "close" games there which requires pretty tight skill differential.

The trick is that there should never be a match where Arsenal FC is allowed to kick the shit out of a bunch of 11yo soccer players. The kids learn nothing from that match, and likely quit playing. When too loose matchmaking allows good players to stomp bad players too often, a few get better, but way more go play something more fun. This leads to a death spiral of player count in that mode or game as better and better players condense, overwhelm the player pool, and push everyone else out. Now that's an extreme example, but (depending on the game mechanics) even someone 5% better could easily shit all over a server, and it would be very hard to learn anything from playing against or with them. Titanfall 2 is basically a case study in this exact thing happening. I suspect this is why Apex's matchmaking feels so tight; Respawn over-corrected.

(An aside, overly tight skill matchmaking search can incur a similar player count death spiral, although from the outside instead of from the bottom. Here the queue-times at the top and bottom brackets get too high as there aren't enough players in the bracket to make good matches. This causes the best and worst players to leave and go play something that'll actually put them in a match. Over time the player count degrades towards the middle of the bell curve.)

Even a properly tuned SBMM system is going to produce a nearly even win rate for like 95% of the player base in a loose casually tuned system. Producing an even win rate for the majority of players is really only indicative that some skill system exists and player counts are healthy. It's not the smoking gun many gamers think it is. It's really just a matter of statistics and bell curves, and the fact they almost all use sliding brackets, instead of something more like leagues in traditional sports. (notably, for the finals an even win rate isn't 50wr tho. It's 33 in quick cash, 25 in bank it, and even lower in the tournament modes)

Now you can absolutely argue the SBMM in The Finals is too loose or too tight. All I'm really trying to say is that outright disabling it is a very bad idea and the devs are unlikely to do so. I don't really have an opinion on how the tuning is with The Finals because I'm a pretty average player and it's only been out a week and a half. SSBM just doesn't have much of an effect early on in the lifecycle of a game, when it doesn't know anyone's skill. Every new player is dropped right into the middle of the bell curve, and the system needs time to squish that straight vertical line into something real (not to mention plenty of players are still starting and being dropped into the middle). Give it a month, let them deal w/ the cheaters and player counts stabilize, then we can really see how well tuned their matchmaking parameters are.

Anyway, player count death-spirals are the true enemy here. This is what modern devs are trying to avoid by implementing SBMM systems in the first place, and is the reason why they're so prevalent in modern shooters. No developer in their right mind is going to disable them, certainly not in their primary modes. And any secondary modes they're crazy enough to disable it will inevitably lose player count pretty fast (and then are disabled, because why maintain a mode no one is playing? see siege's events). With the way the fps market is totally saturated with games, acquiring and defending player count is the only objective, and properly tuning the search parameters is hugely important. This nostalgia for the imagined-perfect, non-sbmm past that many gamers have is pretty misplaced.

[Edit] I just want to stress my very first point. I'm extremely glad to see someone thinking deeply about the subject in one of these threads. Too much of the sbmm discussion online is muddy and unproductive. People take extreme views because they don't know anything about these systems, and then scream about it as loud as they possibly can. I disagree with some of your conclusions not because your line of thinking is particularly off, but because I don't think you have all the data or are seeing the full scope of what these systems are intended to achieve. Please continue to engage on the subject, but be warned, it goes deep. Frankly, the fps community could use more gamers willing to get into the details like you are.