the Republican primary process is designed to produce a nominee as quickly as possible. Hence the preponderence of statewide winner-take-all primaries and party-insider-only caucuses. McCain was able to do what he did this year because of a crowded primary field, and because unlike in previous years, no establishment candidate had emerged. Forcing states to take the measures you describe would violate the Republicans' professed belief in states' rights. This is why many Republican primaries continue to use WTA while the Democrats have mandated PR.
The purpose of this system is to avoid a deadlocked convention, prevent long, expensive primary fights, and give the party's nominee plenty of time to raise funds for the fall campaign.
I agree that the lack of a clear establishment candidate was critical for McCain. He might have been the establishment choice in early 2007, but that ceased to be the case when his campaign imploded that summer. Neither Romney nor Giuliani could take over a clear front-runner position, in part due to those who waited on Thompson's campaign. Huckabee was a late bloomer as well, rising in late 2007 as McCain got back on his feet. McCain was able to beat expectations in Iowa and use that to fuel a win in New Hampshire.
Switching to a system like that used by the Dems would have prolonged the race, but I still believe that by the time the Feb Super Tuesday results were in the race would have again reduced to the same four. With Thompson and Giuliani out of the race, McCain would have continued to add to his delegate lead under a proportional system through March and April and probably would have reached the necessary delegate numbers before the last primaries were completed.
