I am skeptical of Mr. Prusa's reasoning for not being open source. I am not doubting the sincerity of his belief, but I think he's wrong.

The Chinese are very good at cloning. Releasing the design doesn't change this, as they don't need the PCB layout because they already have their own PCBs and have lot of people who can design electronics. In any case, there are hardly any secret sauces on a Prusa. Rather, Bambu doesn't even need to copy and are seen as surpassing Prusa in some respects(true or not).

He mentioned about keeping things secret to prevent supply chain competition. I wish I knew more about this issue, so I can't confirm or refute it, but it seems dubious to me as well.

Anyway, open source is only as good as how you use it or develop it. Prusa seems to be for the most part a closed shop, so they don't benefit from community development and seem to have Not Invented Here syndrome and allergic to using community developed solutions, except for software.

For the majority of people, the open source label signifies that you are much less likely to get scammed or get taken advantages of companies or individuals. That's probably the most important thing.

Anyway, I don't run an open hardware company, so take it with a grain of salt.