First, the big problem... (politely)...

I have no idea how 'mentally fit' he was before the stroke, but his ability to communicate is currently impaired. It might be a tragedy that the stroke affected him, but it takes away a central requirement of a (real) Senator -- in the same way that 'losing an arm in a car accident' would remove a central requirement of being a concert violinist. It doesn't mean that your life is over, but certain avenues are now (unfortunately) gone.

A 'real' Senator must have the ability to 'have a point of view and persuade colleagues', 'engage in useful two-way communication with constituents', 'negotiate best solutions', etc.

So here's the subtle problem...

The political machinery has convinced (some portion of) the voters that the ability to do the job of 'Senator' is not important. All that matters is whether you will 'vote with your party'. This mindset accepts that all of our elected representatives are mindless robots who are simply expected to 'vote for red or blue'. Worse yet, this mindset accepts that someone else sets the agenda. (But who?)

We, as 'The People', should expect that our representatives in Congress are working on our behalf, negotiating the best possible outcome from among a multitude of options. Instead, we have (some portion of) 'The People' who are clearly not interested in (proper) representative government, but simply see it as a power struggle.

I wish they would ask themselves - "What is a Senator's job?"