Starting Strength is not powerlifting, and strength is a general adaptation. The fact that you do not know this makes me think you haven't read the book, and therefore are not in a position to argue against it.
> Few power lifters can do calisthenics staples such as muscle ups, flags, and front levers.
These are skilled movements and must be trained. If you're stronger, learning to do these things will be easier and quicker. (Since they are ultimately strength-based movements, as are most things, and strength is a general adaptation.)
> So, since the barbell method is not the only type of strength
The only type of strength is to produce a force against an external resistance. That's what strength is.
> Linear progression is dangerous. Within 8 months, a beginner will be hoisting hundreds of pounds in their squats and deadlifts.
Good, that's the goal. Their bodies adapted to the external stresses and became stronger. Now they've grown and are physically capable of lifting hundreds of pounds. How is this a negative?
> Not all of them will have proper form
We're talking about four basic movements here. Anyone that is persistent and possess enough intelligence to read SS (and is young enough) can, in fact, do these movements. Certain cases do need a coach, but that's the exception. (Except for very old people--they seem to require coaches.)
Anyway, I don't really understand your thesis: again, the data is telling: weight training is just about the safest form of physical activity. It's also just about the only form of physical activity that lets you precisely, numerically increase the weights such that you become stronger in a controlled, measured way. If you really believe that doing gymnastics (muscle-ups, flags) is safer than squatting, then you're delusional. (And the data strongly agrees with me here as well: injury rate for gymnastics and gymnastics-like sports (cheerleading) is rather high.) If you believe the physical benefits of doing gymnastics exceeds those of weight training, you're extra delusional. (No incremental loading, the lower body consists of muscle bellies that are just too large to load effectively with bodyweight.)
Why do you think that what strongmen do is relevant to your training? Also, why do you think that strongmen are an authoritative source on what to do and not to do?
1. You didn't have "proper form", since a remotely correct squat shouldn't load the meniscus. Additionally, this is just fear-mongering: data tells us that the injury rate of lifting weights is astronomically low compared to almost any other sport. It has the added benefit that it actively prevents injuries in other aspects of life as well. All physical activity involves some risk, such are the constraints of a physical human existence.
2. You need to lift heavy to get strong. This is the only way to induce stress that causes the right adaptations for getting stronger.
3. The novice phase later introduces chin-ups and also of course has pressing and benching. The novice phase lasts--at maximum--about 8 months (when you're a 19-year-old 150 lb genetically gifted skeleton). After this, you become an intermediate and vastly diversify your exercise selection. It's here when you do a lot of upper body assistance and you really don't end up with strange proportions.
I mean, people that do the program correctly and actually do a linear progression on weighted chins don't end up with weird proportions, even in the novice phase. (Although they're not added in until later.)
> But I don't think that dead lifts are the only way to get strong enough.
I think the key to getting strong is barbell exercises, because they can be precisely and incrementally loaded. (So you linearly progress.)
> And core strength helps prevent back pain.
Deadlifts, squats, and presses are all extremely good at developing "core" strength. Ab-specific exercises are not (they cannot be effectively loaded, and the loads are much smaller, and the spinal flexion/extension involved often aggravate existing back injury).
> chair that helps you sit up straight, and provides lumbar support.
If the chair is helping you, then you are not using your muscles and will continue to atrophy away. Lumbar support in general is not a good thing.
Stupid anecdotal evidence here, but my experience with wrist/hand pain from computer use is that it's largely addressed by being strong.
During my senior year of college, I had a lot hand pain and I also didn't go to the gym that year. Now, I type more than I ever have, but I'm also deadlifting over 500 lbs x 5 and I don't get any pain at all.
(Dead)lifting is a remarkably good panacea. Back pain, mental blocks, anxiety. Fixes a lot of shit. If you're not lifting heavy, you should be. [0] is a good place to get started.
The article mentions those grip strengtheners in (general) support of this--but the deadlift already does this (i.e., train grip) and a lot more.