What's the downside of open sourcing it?
I assume regret. Perhaps someone else will take it and finish it release it on steam and make bank. Who knows.
Part of the reason of asking the question is I was hoping to hear from other people who have done something similar so they can share their experience and feelings on the matter.
Whether you open-sourced it or not your next step might be to start building a community to allow for enthusiasts' input.
That would be pretty much of a major change without additional coding required.
This would not be the same game effort from you compared to coding it all by yourself any time you have anything to do with it. Like it has been so far.
You would have to be very careful about the community you keep, but it should be about the opposite of isolating.
Now back where we started, OTOH what if you were simply "done with the game", but the game itself is far from done?
I estimate you've got more major milestones in mind and that would mean a lot more of the same kind of effort.
As you are considering, opening it up could give it continued progress whether or not you contribute any more yourself, also whether or not it's you who builds a community.
But it might be a unique opportunity to build a pretty similar community as if the game was far more complete.
Starting right now without having to wait.
Could be the least isolating your progress has been since the beginning.
And community building or participation could be just the thing for a breath of fresh air, and you were probably going to do it sooner or later anyway.