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Are house designs covered by copyright?

You could always write to him and ask him to specify a licence.

This is great. The more people subscribe to open source, the better it will get for everyone.

I have mentioned examples of games that saw commercial success while being open source. And of course, delayed open source is also an option as some other users have said here.

I think it could be viable for a company to release a game with a “5 year FOSS promise” or something similar

Yes, that is one of the options I mention in the article. But there are games that are open source from day one, such as Mindustry, which have seen commercial success.