I was in this exact situation with a part-flown TATL - I'd purchased the ticket in Nov2019, flown the outbound in Jan2020, and the return was supposed to be in Apr2020, but I cancelled in March2020 because of Covid. I agreed to a FFC at the time for the return - can't remember that the flight was actually cancelled before I cancelled - I think at that time, getting refunds was very difficult. I was worried about when I had to use the return by (nominally Nov2020 under the old rules) and was happy when they extended the expiry. The original itinerary was LHR-YVR return and I was told that it would only make sense to use the FFC for a return trip with the original city pairs - if I tried to use the FFC against some other itinerary, it would not be worth very much. Anyway, finally, this August2021, I needed to fly a single YVR-LHR trip so I called to try to use the ticket. After some consultation with the rate desk, they came back and said, 'yes you can use your old FFC - and you need to pay an additional $1400'. It wasn't even Y class - I think it was in M class. I objected strenuously, and pointed out I could buy a brand new one-way M class ticket on the flights they were suggesting for $1200 without FFC. So they explained exactly as you reported, that the FFC would only work when the return was part of a super-expensive 'more than a year stay' fare. At the end of the conversation, the agent suggested to me that I write into UA and explain the situation and ask for a cash refund. So that's what I did - I pointed out that in all these 'extensions of expiry of FFC' that they granted, they never once explained that in practice, it might render the value useless if you waited more than a year.

Anyway, I received a full cash refund, without any hassle whatsoever, shortly after I sent in my refund request. It was for the full ticket amount as well, not just some partial refund, which I thought was very generous, considering I'd part flown the ticket. I am grateful to the phone agent, because, like you, I was going to throw away the unused half. And I also purchased a new Business class one-way for $1200 on the dates that I wanted (they had some kind of low fare J sale), so United ended up getting their cash back due to my purchase.

I suggest you write in and ask if they would consider giving you a refund in light of your situation.

PS, I used the standard refund request form on the website, because that is what the agent told me to do. (united.com/refunds). I think in the webform, if you try to put in your ticket number or record locator, it gets rejected (because as a part used ticket it's not eligible in theory). So I filed the request under the 'other' heading , and then included the record locator and 016 number in the webform and also in my typed explanation.