I’ve definitely encountered this problem, both with games and with physical letter writing in general. There’s definitely an added fragility to letter writing games. A normal tabletop game doesn’t run the risk of ending entirely if someone doesn’t take their turn in a timely manner. Maybe a game could be built that can’t be busted by somebody forgetting to send one or two replies.
What if there were specific, predetermined days and/or times when each player was allowed to send their letters, so that missing one deadline just meant waiting until the next one. It doesn’t become some annoying thing hanging over the procrastinating player’s head, because they literally can’t do anything about it until their next window comes along.
Bonus: it lets the designer plan out more of the broader narrative arc.
Viewing post in Making letter writing sustainable
For posterity’s sake, I wanted to note hear that my entry, Loss of Signal, was essentially a test of the above hypothesis. All letters in that game must be sent within specific predetermined windows, and failure to do so results in in-game mechanical shifts. Therefore the game is not necessarily derailed by temporary forgetfulness or a dip in interest. It provides its own opportunities to pick the story up again. Have I actually tested it? Heavens no. But I look forward to finding out if it works.