The list of possible ending should include quit for personal reasons.
Or at least, reasons not directly connected to the game. My one and only PBEM experience was a newbie vs. newbie game against Malagant, he as Japan and I as Allies. The game got only about 5 months in, to mid-May of 1942, when he vanished from communication. But the Japanese were dominating my forces easily pretty much everywhere, and as far as I could tell they were in a very good position. I could only guess that he had some sort of personal circumstances that came up; perhaps even a tragedy, but I never knew. In any case it would be grossly dishonest for me to count this as a victory even though my opponent did quit.
I respectfully disagree. An opponent that drops a game without communicating the reasons why or offering an 'out' for another player to step into his stead is breaking the pact. Quitting like that is tantamount to admitting defeat. It's certainly understandable if they have a terminal health problem, but-in your case-it's far more likely that the person just became bored with the game and 'ghosted' you. I consider that sort of action to be extremely disrespectful and it's worthy of putting them in the box of 'losers'.
How hard would it have been for your opponent to send you an email to clarify this? After you spent hundreds of hours on the game, he just disappears? Seriously, how long does it take to send an email saying, "I'm not having fun with this game. I'm sorry, but I can't do this for another 4 years. I resign. You win. Thanks for the game." Again, if he's sick and / or dead, I get it. But the vast majority of games ending aren't because someone dies. Unless that happens (it's happened three times to me), they have no right to say anything other than 'they lost'.
I would hazard a bet that if someone drops a game AND drops off the forums entirely, a catastrophic life event has happened. As you say, pretty easy to send a note. But how many spouses would know to send it. How many have written instructions to do so in the event it would be needed?
Now, if you see activity on the account, that's another thing. But an account going completely inactive, that's not just a game drop. Just my opinion ...