For me, that would be a keeper, even if I were to upgrade. I have several coins of which I have several specimens in different conditions. I don't generally collect to fulfill a mission (ie, finishing a set). I collect/accumulate for self satisfaction.
I have 3 of this coin, all in low grade and they all have readable dates and mint marks. What’s one more?
Same. I have 5 1900-S dimes, raw and damaged through MS65. Why? Because I discovered a reverse transition variety about 5 years ago. I sort of think of it as "my coin", so every time I see the scarce one I'm tempted to get another. It's crazy. Someone help me. Other times I think people have been cornering the market. There was a period a few months ago where there were two dozen 1901-S and 1913-S quarters (key dates) for sale on Heritage. Kinda seemed like they were all from the same hoarder.
You said it yourself, you are attached to it. Being as such, keep it. My very first set as a teenager were Mercury Dimes. I put that set together with my grandfather. Yes, there are low ball coins I could now upgrade as I have a professional career. Yes, there are some that are flashy, and clearly cleaned or damaged, but I keep the set as is. I just feel transitioning it to a more cohesive and uniform set with higher graded specimens would just pervert the memory of making that first set with Grandpa. In short, if that coin still makes you happy, keep it.
It all depends if you can get past the problem and can afford better. The 1916-D Mercury Dime is much more expensive that it was two decades ago. The last time I handed one, it was for a want list customer who wanted a piece with all of the ax lines, a classic VF. Naturally I had to buy one in an EF holder to fill the order, which I warned him would happen. Still it cost less than $2,000. Today the same thing would probably cost $6,000 or more.