Since you can't fix a broken string just by removing a link and closing it, the only protection against string breakage is to carry a full spare.
Your definition of "serious cyclist" excludes a lot of serious cyclists. Precisely because it happens so rarely, carrying a bulky tool for a rare problem isn't worth it to everyone.
We've seriously shifted the goalposts from "serious cyclist."
Folks who are serious about any hobby or activity are also often relentlessly practical. I used to ride 100km from the city I lived in, out to my brothers farm. It was not a particularly long or challenging ride, and it had cell service most of the way, even 20 years ago. I always did a gear check, and carried spare tires, but in 4 years of doing that 20-30 times per year, I replaced tires, but never a chain on that ride. I also never carried the tools to do that work because it fell into the category of unnecessary gear. If there was a problem, I was only about a 30 minute wait for help from either side if I needed to be picked up.
I also used to do a fair bit of trail riding, and for a much longer period of time, and for significantly technical trails. Again, a gear check ahead of time, and proper maintenance meant I took tubes, but not the chain repair tools (and one time I regretted not taking 4 tubes, because it was a long and hot 14km hike back carrying my bike on a July afternoon).
I am not going to gatekeep your ability to gatekeep, but you should at least understand that your perspective on what constitutes a "serious" cyclist is completely subjective and not rooted in anything meaningful.
There’s a tendency to ascribe value to words like “serious” but it’s a temptation, nothing more. Who cares that some bozo thinks “serious” drivers drive manual transmission cars, or ride bikes in a particular way.
The worst mechanical I ever had was a rear derailleur which deformed under load and pulled into wheel. I have never heard of anyone carrying a complete spare derailleur unless on a round the world trip.
https://www.decathlon.co.uk/p/900-bike-multitool/_/R-p-10042...
[1]: http://www.birzman.com/products_2.php?uID=2&cID=31&Key=54
Edit: there's also two strings, one on each side. So if one snaps, I think you can ride home with the other
If you put a new chain on old sprockets, it will likely skip, particularly on the rear cassette. This will go double if you're someone who allowed the chain to wear for so long that it just broke. That chain will only work with the sprockets that it is on and vice versa.
If you damaged a newer chain due to some freak accident (like cross chaining while shifting under load or something: shouldn't happen on a quality, well adjusted drive train) you probably don't want to replace it; aside from the bent/broken link, it is good. Splice the damaged link out and get moving.
Swapping in a new chain could be extra work. It likely has excess links which you have to remove and set aside, and may have stuck links that have to be identified and massaged into releasing.
For MTB especially, it’s not the fact that there’s probably better drivetrain options, but the fact that the current options will happily take a beating, and still let you ride back out. I’m sure the string is real nice and quiet, and has a nice internal hub, but my chain, mech and cassette will get whipped into trees and rocks, will have sand, water, grit ground through it for ~20kms, sustain impacts from going off drops and jumps and they’ll continue to work. I’ve bent mechs, hangers, dinged cassettes, ground chainrings, and they’ll put up with an astounding amount and still get you home.
Once home the ease of access make maintenance a lot simpler, everything gets a clean and lube and it’s happy to go again.
The people I know who've had breakdowns tend to be physically a lot stronger than me, and more "adventuresome" in their choice of riding conditions. For instance I expect my mountain biking friend to have a new story of destruction every few weeks. In some cases, I suspect that neglect or lack of maintenance skills played a role.
I bring more spares and stuff when I go on really long rides, or if my family travels to some remote place with the bikes.
Small distinction I agree, but you come off a little arrogant with such a strongly worded statement.
But as with all riding, it depends on use-case. For remote cycling, you'll be carrying more spares than commuting, where worst-case you can walk the bike home.