There is a big problem with this picture. There are several states which should have 2 GOP senators but have either one or none. Unfortunately, they're nearly all small states. The pool of viable Senate challengers, when an incumbent is running, is small in any state. In a very small state like the Dakotas and Montana, it's tiny. It may be only one person. There is a pattern in recent years of the best possible Republican -- perhaps the only viable Republican -- challenger chickening out.
Partly, this is a problem of state size. But it's also a problem in Republican psychology. My feeling is that Republicans in states distant from Washington don't have much taste for Washington. They don't expect to make much of a difference there with Harry Reid and Ted Kennedy exercising almost complete veto power in the Senate, and they don't like the idea of being in such an alien city, largely run by politically alien people -- bureaucrats and liberal interest groups. Conservatives also don't have the same enjoyment of politics. They don't enjoy it for its own sake as much as the Rats. They are more inhibited by personal friendships (such as the "we're all friends" attitude typical among small-state officeholders, which people like Reid and Tim Johnson exploit very effectively).
They are more devoted to family, and Republican families often aren't very supportive of political careers. I could go on, but you get the point. We're talking about a small number of people in a small number of states. It doesn't appear to be a very impressive pool. Marc Racicot, John Hoeven, Mike Johanns and Jim Gibbons (MT, ND, NE and NV) are just a few of the potentially powerhouse candidates in small red states who have refused to take the plunge. Some of our incumbent senators in these states, like Mark Andrews, Larry Pressler, and Jim Abdnor (ND, SD and SD) have managed to lose races they should have won in recent decades. Conrad Burns is at least an even bet to lose his seat this year. Not a pretty picture. I agree that it could change. But it will need to, and I don't see it yet.