This is something you see often nowadays, especially on social media. Activists get some controversial policy approved, or censor some already-existing content. Yet if you protest it, you get the "dude why do you care" fence-sitters. They'll tell you that it doesn't matter that much and that both sides are silly anyway. Often these people are in complete support of said policy/censorship, but if you point it out they'll just tell you that those extremists don't represent them (because they're the enlightened "centrists", of course), and you get nowhere.
I feel like this is a big problem. How do you deal with this?
Present them with a thought experiment: they can will a good side into existence right now, if they can only imagine it. What does it look like to them? The gamble here is that they will be unable to describe it, and a weak description should be challenged so they're pushed into a defensive posture.
Typically, it is foolish to answer this question because it is rarely asked in good faith. If you were to answer it directly and honestly, you should expect to receive attacks against whatever you answered with (because your answer would be like a giant cartoon bullseye to them). Knowing this, it's not hard to understand that the person asking really does care and they're just a bad faith actor.
But this doesn't answer the question. How to deal with them? Well, I wouldn't bother on a platform like twitter that encourages poor communication habits. I'd probably give the troll-ish Umineko-inspired argument "without love you can't see it", which translates to "you can never understand a thing if you don't care about it". The way you present it can be done to infuriate them by suggesting they're inferior to you for not caring (it may well be true). Using the same logic, you could also just throw their question back because it's a fair gamble that they do not care about you or your motives enough to listen, but I think this is less effective.
I want to point out that this short question is a highly effective bad faith strategy. It is not an easy question for most people to answer, but you naturally try to answer it after reading it. Once the answer proves non-trivial to produce, you take a tiny amount of psychological damage/irritation, which causes you to hesitate in responding.
Of course it's a short-hand communication platform. Hmm, how about berating them for nihilism? The normal defense against an accusation of nihilism is to show that something matters to you, so you then get to take the role of bad actor and attack whatever they answer with (you must be prepared to attack holy cows like BLM).