I seem to remember reading that MI2 sold more copies in its first month than MI1 sold altogether, so I don't think management there was disappointed with the sales. I also don't think anyone was "stopping" anyone from making MI3, either, I just don't think anyone had a strong idea for what they wanted to pitch. Plus I don't think Ron would have considered staying at LEC just to work on MI3 when he obviously was yearning to have more control with his own company. Setting up his own company with Shelley Day was likely far more important to him than coming up with a fully fleshed out idea for MI3.
When I got to meet Ron in London a few years ago, he told me that he started work on MI2 before it had even been greenlit by management. ("It's better to ask for forgiveness than permission" is how he put it.) So going straight onto a THIRD Monkey Island game after that was probably the last thing he felt like doing, too.
Finally, I think people make too much of MI2's ending. Bill Tiller (who admittedly isn't always the most reliable) says that it was always Ron's idea to end MI1 with his "child's dream" thing, but Tim and Dave managed to talk him out of it. When they got to MI2's ending they didn't know what to do, so they relented and let Ron have his mischievous ending... but then tacked on a few things so that someone could make a sequel. I really doubt that Ron had anything deeply or seriously planned for MI3. Maybe a couple of notions, but I think it's mostly a bit of mischievous myth-making, which is why he's never revealed it.