Takes like these do not account for the value you gained by using the software in the meantime. Here are 2 scenarios:
1) company uses exclusively free software, spends more time dealing with the shortcomings of said software than developing product, product is half baked and doesn't sell well, company dies.
2) company uses proprietary but cheap/free (as in beer) software that does the job really well, focuses on developing product, product is good and sells well, company how has a ton of money they could use to replicate the proprietary product from scratch if they wanted to.
A purist approach like in scenario 1 leaves everyone poor. A pragmatic approach like scenario 2 ends up earning enough money that can be used to recreate the proprietary software from scratch (and open-source it if you wanted to).
In this case the problem isn't even the proprietariness of the software, it's the fact that companies are reliant on someone else hosting the software (GH being FOSS wouldn't actually change anything here - whoever is hosting it can still enforce whatever terms they want).
FOSS alternatives already exist, it's just that our industry is so consumed by grifters that nobody knows how to do things anymore (because it's more profitable for every individual not to); running software on a server (what used to be table stakes for any shop and junior sysadmin) is nowadays lost knowledge. Microsoft and SaaS software providers are capitalizing on this.