Protesting is very important, as is visibility of protests. However, I don't believe United fans would be going to the lengths of attempting to prevent the game happening if they were within 5 points of City right now - which was entirely possible this season. I'm not suggesting that lack of success is the reason for the protest, but that possibility of league success this season would have changed the form of it.
That - plus the potential danger it created for club staff, stewards and broadcasters (innocent bystanders who did the safe thing and didn't resist) - is why forcibly entering the stadium on a match day during a pandemic was too far in my opinion. The idiotic behaviour of the minority of those who did enter the stadium also undermines the protest. The media is already more focussed on fan protests than it has been in a long time.