I see this argument all the time. And yes, you are correct: if you don't like it, don't do it. But people seem to forget that 'If you don't like it, don't do it!' too much results into people getting bored completely because there will be nothing to do even faster. Then the inevitable 'Well you don't like the game anymore, maybe you should take a break!' comes. Why would anyone return to a game after their friends have arguably gone away from the game, the content remains of the same structure in a slightly different texture and there is no transparency on what the future holds?
And worst of all, this argument is solely customer-to-customer, or player-to-player. You're essentially telling people to stop playing the game and do something else. If I was a designer, I'd be pretty curious where I went wrong if straight after a content patch, regardless big or small, my own playerbase is telling each other to stop playing the game. And then have it happen a few times in a row too. Shouldn't you be advertising the game for me? Telling others how great it is and defending it with something stronger than 'you know, you can also just leave it be'? Did I make the content just for people to 'let it be'? Depends entirely on the target audience, but then if the audience is that small I'd put some very big question marks behind that.
Basically, yes, it is a valid argument. But it has no substance into it, at all. As a player, it doesn't tell me anything about what's to come and doesn't provide me with anything more, leaving it entirely up to the designers. As a designer, this argument is literally useless: it only provides the feedback that people are.. not doing the content. Well, could've done that with a stats tracker running on server data anyway. Oh, and that they are telling each other not to do content. Even someone saying 'It sucks!' is more valuable feedback at that point (and likewise, 'I like it!').