As far as I know the deaths of the other congressmen-especially that of Gilmor-came suddenly.
I don't how you can impose a standard like that on candidates though.
Paul Tsongas died of cancer several years after he would have (theoretically) served out his second term.
Lane Evans has Parkinson's, IIRC, but he's been able to maintain his health well enough to do his job-although I personally feel his work is to the detriment of his constituents.
I do agree that keeping a congressmen or senator in office even when he's physically and/or mentally incapacitated makes a mockery of our political process.
Lane Evans retired in 2006.