Currently, Romney is getting about 25% of the Republican vote in the primaries and in most of the polls. That means 75% of Republicans are voting for Not-Romney.
If the rest of the vote is divided more or less evenly between Newt, Santorum, Perry and Ron Paul then the those candidate will end up with less than 20% and will lose to Romney.
If Perry drops out then some, but not all, of those votes will go to other mostly non-Romney candidates. If this doesn't happen then the liberal Romney will be the one to represent the mostly conservative Republican party in November.
Accepting your argument, then if Rick Santorum drops out there will be even MORE votes to apportion among the remaining conservatives, dealing Romney an even more pronounced defeat.
So, will you add Santorum to your call to drop out?
Or, alternatively, Gingrich?