No, it will stay as a sub; I plan to just keep adding content/features indefinitely, anyway. You are right that the main story is almost done, but there's more side content and features I have ideas for.
Also, it's totally fine to only sub for a month or two -- I designed it to be easy for people to sub in and sub out conveniently. I know a lot of people will only want to play for one month out of the year to check on the new stuff and then go play other games and not check back in for another year and let the new content build up in the meantime. I do this myself sometimes, too, so I understand. I still appreciate the support regardless :)