This is missing the point, I think. In "concern trolling", "trolling" is doing a lot of work. The purpose and effect of the in-some-sense-insincere concern matters. And obviously some people will use the term over-broadly, but it's describing a real and very annoying tendency.
A pretty big problem with discussions on the internet is that they're full of idiots who think they're geniuses. There are probably a lot of reasons for this. What's so bad about this is that people who think they're much smarter than the people they're arguing with don't really take the other side's arguments seriously. They've decided from the get-go that the point of the discussion is basically to improve the other side - the other side just needs to be educated or convinced or whatever, rather than engaged with as if they have something of value to say. People who think this way come into discussions and do what a lot of people would do in this situation. They don't really participate in the discussion as holders of a particular opinion, but instead try to help people along and nudge them towards what they see as better opinions and better ways of arguing. Often they even present themselves as basically agreeing with those they're trying to educate, perhaps because they feel like that's the only way they'll get any sort of hearing at all.
The problem is that, as stipulated, they're actually idiots. They're not in a position to manage the education of the other side. They raise stupid concerns, which almost nobody who actually believes in the other side would raise. Perhaps the concerns are obviously baseless, or perhaps they're actually something that's been discussed often before and so have been dealt with, or perhaps they're essentially concerns that doing anything significant to advance the movement would be inappropriate.
If a well-meaning idiot who actually believes in whatever-it-is has concerns like these, there's value in talking them down. But when it's an idiot who's doing this either just to troll or, more charitably, out of this misguided desire to improve others that I describe above, there's basically no value in engagement. When people are defending positions that they're not actually very invested in, which is what the concern troll is going to be doing, they're often more-or-less invulnerable to learning. Their arguments can be torn to pieces and they'll use exactly the same ones the next week to argue with different people. And even if the concern troll is convinced, nothing significant has been accomplished - the concern troll doesn't share the original position which they were concern trolling about.
So even if you want to say that actually it's often the people claiming that others are concern trolling who are the idiots, such that maybe half the time the concern troll has a point, discussions with concern trolls are still going to be about 50% less likely than sincere discussions to bring someone to a better-thought-out position. A norm of sincerity in internet arguments is a good way to raise the signal to noise ratio. Not because advocating a position that you don't actually hold or care much about is always a bad way to engage, but because you need to be capable enough to recognize when it's useful and appropriate, and relatively few people are that capable.