The more that make a career out of it, the more will age and die in the office. Frankly, I think that anyone who seeks office knowing they are terminally ill enough to likely not serve out their term is doing a disservice to their constituents. Sorry this Congresswoman died of cancer, but knowing that she was going to, she should have gracefully stepped aside.
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.