Nothing to lose is maybe not correct. I've been in kinda the same situation before and chose not to open source my app. This was years ago, when I stopped with the project the number of active users had already been declining for years. Somebody contacted me to take it over, but I feel like he wasn't serious about it. He just wanted to keep it running and be the owner, but not do any of the things needed to get it back on track.
There are a couple of reasons maybe not to open source it:
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No code is perfect and people can get really self aware about that. I know I have had imposter syndrome in the past where I thought my code was shit, but people always complemented me on the result. Opening up the code can lead to people seeing how shit it really is and call you out on it. The code is probably fine, but it's a legit fear.
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With open source people expect you to provide a way to report issues and respond to those publicly, with many people adding their 2 cents and big discussions. If you don't see it the same as they do, or your solution isn't what they expected or you simply aren't fast enough, it's a problem. We've all seen flame wars and stubborn developers on open source projects, the drama factor is real.
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Forks. Simultaneous the strongest and weakest part of modern open source software. On the one hand it's awesome we can make multiple versions if the needs diverge or the original devs abandon the project. But on the other hand, it leaves you with very little control over the project. One case I've seen one dev worked on a project almost every Sunday as a hobby, the project was popular and had a tight community which grew a bit big. The amount of open issues grew and bigger tasks just didn't fit in the original dev's schedule. One of the community members was a good dev who just got laid off and had some extra time. So he forked the project and spent some weeks working on it fulltime, fixing a lot and expanding the project. The original dev wasn't happy, it was his hobby and the community fragmented over people using the new version and the original version. Everybody was upset and the original dev called it quits and said talk to the new dev. The new dev however got a new job and also abandoned the project, with nobody left to pick up the pieces. The community was already fractured and people stopped using it.
Of course these are only some examples and people feel different about different things. But I can understand reasons not to open source stuff. I also haven't mentioned the many many reasons why it's a good thing to open source stuff, they exist and are valid as well.