Why MO is dependent on the UO:
How many Modern Orthodox Jews know how to make tefilin? And I don't mean intellectually, I mean knowing the ins and outs of their actual manufacture.
How many Modern Orthodox Jews are skilled enough sofrim to write a whole kosher Torah (or any other ritual scrolls for that matter)?
And besides for just knowledge, how many MO would be willing to invest a large portion of their lives towards becoming skilled in all those ritual items that they get UO rabbis in Israel to make for them?
What percentage of mohels or shochets are Modern Orthodox?
Now, maybe it could be that if there were no Haredim around, the MO would fill the vacancies in those necessary positions on their own. But as things stand today it is the Haredim who hold almost all the knowledge and all the experience in how to produce these things that MO Jews take for granted. I, for one, hardly even know how to braid my own tzitzit.
Worldly, cosmopolitian and successful Modern Orthodox Jews may look down their noses at the Ultra-Orthodox for their backward ways of thinking, voluntary ghettoization, and their weird, sometimes scary, religious zeal, but without the UO from where would your average MO parent getting ready for their son's bar mitzvah pick up a pair of tefillin?