I had no complaints at all with my gut/poly the entire 12 weeks it lasted. If anything when I first used it fresh it felt too powerful and I wasn't sure I liked it.I often wonder if there is an issue with confirmation bias in any of these assessments. The player knows tensions change with string bed aging and he knows how old his string bed is. So every bad shot on a new string bed is attributed to user error and every bad shot on an old string bed is attributed to the strings. Then there is the confidence issue, once you start to be convinced that an older string bed is less reliable.
I think an interesting study would be to blind players to the age of the string bed and look at their match results over several time frames. I suspect the match results would not be altered as dramatically as players feel.
Fortunately most of my competitive play comes in tournament doubles so I just re-string a frame prior to the tournament and use it through the tourney which is usually 2-3 matches. I'd probably suggest the OP have a "match racket" that he uses for at least 4 matches and nothing else. Then convert it to the practice racket and restring a practice racket to become the "match racket".
I much prefer old gut/poly to old full poly.
Gut/poly is not something I ever plan to cut out prior to it breaking because it's 'dead'. I will just play it until the mains snap.