Yes indeed. So?
The cost of substitution for such scenarios is tremendous and you haven't even begun to add any 'beneficial' mutations yet.
Son, once again, there is no "cost of substitution" for neutral fixation. Period. Deal with it, and stop repeating falsehoods that don't get any more true the louder and more often you repeat them.
You haven't a clue what you're talking about, you're just flinging buzzphrases around with no understanding of when they do and do not apply, but that doesn't stop you from blathering on about them endlessly. Why do you behave this way?
It is simply ludicrous to believe such scenarios.
I'm sure it looks that way to someone who doesn't understand them in the least. But they make fine sense to those of us who do.
Now neutral fixation is a different thing that a single-infection moving to fixation.
Single-infections moving to fixation follow the same path as beneficial mutations (according to the claim) even though they are not. This makes their fixation even more problematic, not less.