The assumptions here are that your secrets give you an advantage by being secret, that they're easier to steal than to rediscover, and that that advantage is necessary to success. These assumptions are not universally correct, and I suspect that their degree of correctness is rather strongly overestimated.
Founder groups would have the same problem in pre-formation situations. Say, four founders build something that they have worked on for a full year and are prepared to launch. One of them defects and says to the group, "I am going to take everything that we have worked on and take it for myself." Of course, that is outrageous. Buy why? Because laws exist that declare it illegal for someone to misappropriate what the founders have been working on. Those are IP laws. They protect the interests in intangible property. Without them, every founder would be vulnerable to such defections, without any form of legal recourse.
That doesn't sound like any of the standard copyright/patent/trademark/trade-secret categories, it sounds like it has something to do with (probably implied) contracts.
Open source is no exception. It relies heavily on rules of copyright law and on licensing to make its system work. If everything were freely shareable without any form of restriction, one does not have open source - one has freeware.
So then Postgres, SQLite, the BSDs, clang, etc are freeware instead of open source?
Startups depend heavily on IP laws. Such laws have great value in today's startup culture and ought to be recognized for that contribution. Reform them, absolutely; abolish them, don't even think about it
I don't think that's a sound argument, we've done just fine abolishing other practices that businesses seemed to depend on. What is observed to happen when/where such laws don't exist or are generally ignored?