I only read the full thread if I want to, if by some chance it has become interesting. Usually, I'm interested in the opening post and my response to it. I don't actually care what a load of randoms think, so why would I read?
You do realise that, to everybody else, you are also another 'random', don't you? Are you expecting them not to care what you've written and thus not bother to read it?
If you only ever want to see an OP and then consider your own instant response to it, without bothering about other people's opinions nor expecting them to bother about yours, then why ever would you consider posting at all? You might as well just lurk, read the OP (or anything online) and then keep your thoughts about it in your own head - unless you assume that anything you might have to add must automatically be of huge interest to everybody else, who themselves are all without a doubt posting what must surely be meaningless rubbish and guaranteed not to be worth a moment of anybody else's time?!
The whole idea of MN is for 'random' people across the world to discuss a whole variety of issues and to interact with each other. If a thread gets (in your perfectly valid opinion) too long to be of interest, then just keep on scrolling. By adding your own comment, all you're doing is making it even longer.