No...you are just ASSUMING that all the voters/supporters of Gingrich and Santorum hate Romney. And that is not the case. And that also is not how most people ACT. Most people don't strategize that deeply. They will either not bother to vote, or they will pick their next choice, even if it means hurting their top choice. Especially Gingrich supporters who mostly realize he is out of it.
I am not saying that Ron Paul won't pick up ANY of those undecideds. Just that people who think he will get a big majority are way off. I'd say an OPTIMISTIC scenario would be about a third of those undecideds going to Mitt Romney, a third going to Ron Paul and a third not voting. But more likely, the percentages will stay pretty close to the same, with Romney picking up about 60-65% of those undecideds (not counting those that don't vote) and Ron Paul getting about 30-35%
No matter how many people call these undecideds, a large portion are not going to vote. Unless there is some important local election that would make them want to vote, like a mayor that they want to support or get rid of, many will stay home. (And by the way, it is highly doubtful that Gingrich and Santorums campaign are going to spend money and time trying to GOTV for Ron Paul. So it would not be a failure for 2 of those 3 (or 4) if these poll numbers hold up.)
And like it or not, Ron Paul's poll/vote problems come from the fact that the age group of his main supporters is the age group that doesn't vote. Sure..they go to rallies. But they go to rallies for the spectacle ad to be part of a crowd. Hell. my freshman year they were going to demolish my dorm that summer. Most of my floormates HATED the dorm and wanted to move to a different one the next year. But they still staged a protest in the presidents office even though it was AGAINST their best interest since everyone that would have been displaced from the dorm would have had a much higher priority when it came to getting room assignments for the next year. But they did it because it was a big party. And even when I pointed this out to them, they said they didn't care. It was fun. (At least 4 of them ended up NOT getting into the dorms they wanted when they would have if our building hadn't been saved from demolition.)
Whether people want to accept it or not, half the people who show up for those rallies have no intention of voting at all. Probably aren't even registered. And of the half that DO intend to vote for him, probably at last half forget when the day comes, or are hungover, etc.