I could argue against the "The only strains of Betta with that intensity I would say are the Thai-bred fighters" statement. I have a few of my own (granted they don't make up a big part of the overall population) that aren't technically "thai bred fighters" and they will easily and quite happily tear another fish up in the blink of an eye. I had my female CT Evey completely tear my late CT female Millie up after a divider accident and I ended up losing the other female. Granted, I've also seen bettas be really really aggressive in their own tanks flaring at fingers, etc but do phenomenal in a divided tank. I haven't tried a community myself.
I am sure there are bettas here and there that are exceptionally aggressive and I totally agree that Bettas are not docile fish but I would say there are even cases of exceptionally aggressive guppies or goldfish that harass their tankmates to death exists. I have had Tiger Barbs nip tails to the bone. Some of it could also be acclimation to tank mates and a proper setup, I believe the majority of bettas are not as aggressive as people make them out to be and would fair pretty well in a good setup.
Plakats and short-finned females would certainly be capable of catching fairly fast-moving fish. Even long-finned males can have surprisingly quick bursts of speed. Plus, the betta doesn't have to actually catch another fish to cause it a great deal of stress.
What you have to watch with bettas is their unpredictability. They can have very abrupt changes of mood, and what they tolerate one day, they may not tolerate the next. You can often see this in young females living in a group setting. They reach sexual maturity, and as their aggression and territorial drives intensify, they suddenly start squaring off with fish that they previously cohabitated peacefully with. I see the same thing occur with my wild bettas, as the young fish grow-out alongside their siblings and parents.
There's also no way of knowing how a betta will do in a community setting, without actually putting them into one. I've read a great many posts on this forum, where a hobbyist has set-up a community tank and introduced a betta, only for it to immediately start trying to kill the other fish in the tank. So now the hobbyist is left to set-up a second tank for the betta, and you can see why they would be wary to try again, or recommend such a thing to others.
You make very valid points but I don't quite understand the unpredictability part, I have never encountered a case where a Betta whose mood swung that drastically or heard of someone with a community tank whose Betta suddenly become aggressive overnight, at least not lurking around the forums. Are we talking about very young fish, like fry that reach adulthood in a community tank? I only ever worked with adult bettas so I am not sure how very young bettas would work, so it could be that, perhaps bettas are not suitable at certain times of their lifespan.
It is true, you risk having a exceptionally aggressive fish by sheer bad luck, I would say that risk kind of exists with just about any fish. Sometimes you get a nasty apple, I had a Tiger Barb that nipped other Barbs to the bone, there was nothing I could do but separate and eventually get rid of it. I would also cautiously wander how a introduction was done and what the setup was if a Betta was killing everything. Tank size often plays a role, if I was a Betta, cramped in a small tank with no personal space, I get pretty feisty too. I also don't think there a is a ton of info on how to introduce a Betta to a community setup, I am no pro or anything, I would say there needs to be a acclimation period where the Betta can see but not harm the other fish, enough plants/decor to break line of sight and enough fish that no single individual is singled out. Granted, I'm sure there have been failures even when everything is done right but I don't believe a few bad marks ruins a painting.
My own experience the last few years has been that they are good community fish and the Betta are more lively when kept in a bigger community setups, I haven't had a tailbiter since and they are definitely more active. I'm sure there are people who are just too nervous for community setups and because a Betta does not require company, its not a must. If the tank is small, I definitely say no, they are still the kings of desktop buddies. I only had bettas do well and I genuinely believe a best setup for a betta is 10gal or greater planted community and is what I always encourage people to establish. But, I am curious however to learn about cases where it has failed, maybe it wasn't planted, maybe the type of tankmates, you know, so I'm not setting people up for failure. Maybe its also a management thing, perhaps someone who doesn't have a ton of time doesn't want to scan a entire tank to make sure every fish is a accounted for, etc.