Personal question to programmers

Imagine the situation:

You are developing a free open source program, the purpose of which is to become an alternative to paid and reduce its market share. (While the absolute leader paid.)

Your program, of course, for purely objective reasons is much better than a competitor. But in order for all these advantages to appear in full, you need to save files in your own format. A standard de facto - the format of a competitor. Bad and not quite open. And yours is good and take-not-want-open. It is clear that you need to work with the leading format, and with your own, and your program knows how.
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Question:

In what format will your program save the default files? (In your own or in a competitor format.)
Why? (Be sure to explain.)

Then I will tell you why I am, and give my version of the answer with a rationale.