United fans on this forum are a bunch of toddlers who I have a hard time agreeing with about anything, and I say that as one of them. Honestly, you'd probably find that most of them moan about water being wet.
I don't think anything said in that thread should be taken as the gospel opinion of fans. Every player has bad games or a dip in form, even WC players. But as soon as it happens, you better believe someone will be on here posting something reactionary, completely forgetting about the 10+ games before that. It's just silly.
I don't know whether Shaw is WC, but I guess it's all relative? At his best, which has been more often than not the last few seasons, even when the team was struggling, there aren't currently any LBs significantly better than him, if at all. If you swap Shaw with any of them, that team at best gets better, or at worst stays at the same level of footballing quality. His style, strengths, and weaknesses, may differ from said player, but his qualities in other areas are high enough to make up for it.
That's the most diplomatic way I can think of putting it.