From their most recent post here
"Also for fans of My Dystopian Robot Girlfriend. The game currently is below the thresholds, so we will survive. I got a bit scared at first though. If we ever reach the 1m$ yearly revenue threshold though we will be forced to switch to a paid model. Otherwise we'd have to pay thousands of dollars to unity and we'd go bankrupt."
Also, from this thread about a similar topic
"Moving engines mid project is not an option. It'd take months for a most likely inferior product."
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I highly doubt this game will die because they use Unity, and the devs have a plan for if they end up reaching the Unity monetization thresholds