That is not necessarily true.
Murkowski lost her primary and may still win the Senate. Lieberman has already done it. Losing a primary doesnt make you a weak general candidate, just makes you a weak party candidate.
If a Republican cannot win a primary, then yes, it definitely means they are a WEAK candidate as far as being a Republican. Now, if you prefer to have candidates who are strong with centrists and Democrats, then I agree with you. Murkowski might indeed win against the votes of conservatives. Since this is a conservative web site, I assumed you wanted a conservative to win. Plus, Joe Miller would have almost certainly won the general election had Murkowski NOT entered the race as a write-in candidate. Again, posters here should find that the most appealing outcome, since we are supposedly interested in conservative candidates.
O’Donnell is a different situation than Miller of course, but she should have been enthusiastically supported by the ENTIRE party. She was kneecapped by the moderate/centrists. I’m not going to say Rove cost us the election, but I think it’s safe to say the negative PR from her fellow Republicans certainly hurt her. They would have expected us conservatives to unify behind Mike Castle.
Finally, the fact that incumbents are so strong is exactly what is wrong with politics today. The founders never ended for people to be senators for life. Yet, incumbents in both parties are using the power of their position to serve special interests and hold onto power. To do so, they frequently violate the constitution. Again, that’s something that people who post here are supposed to be concerned about.