Especially bearing in mind that United almost certainly voted in favour of the proposals. It's been alleged that the voting threshold had to be reduced on the day (from the usual 90% to a more amenable 75%) to enable the proposal to go through after very vocal opposition. Even so, the great majority voted in favour, including all of the "bigger clubs" I'd wager.
Any thought that United, as a club, will be in some way "disrespecting" the competition (by, say, playing the u18 side) is pie in the sky.
Opposition will necessarily have to come from fans: Third and Fourth Division clubs are by & large willingly dictated to by the premier league clubs. The exceptions to the rule, of course, being led by the supporter-run clubs like Wimbledon and Portsmouth.