If it lets people do what they want to do without limitations over comparable engines then there's no big requirement to go open source.
However it can help attract more users. There's a general preference for 'compiled in' solutions - if you stop developing / don't support some new desired feature, all projects that use your engine are affected too. It's forcing a dependency without any guarantee that you'll support peoples projects in future. Potentially people could be putting years worth of work into a project so future viability of their dependencies becomes important.
Plus assuming you have a big enough user base you'll get bug-reports and submissions that'll accelerate the project - so you'll be able to do more yourself than time currently lets you.
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