I agree with you that the open source software has higher level of trust than the proprietary one, but I afraid I just can't maintain Marta steadily for a long time if it was open-sourced (and then forever free). And I doubt the community will do that instead of me. In case if you're interested, I expressed my thoughts about this here [1] earlier.
I do promise, though, that Marta will never collect personal data, display ads or do some other bad things. I just don't see any sense in that, and I'd rather have a good reputation.
[1] https://github.com/marta-file-manager/marta-issues/issues/19...