Well, this thread is getting off-topic real fast but okay, I'll reply :)
This argument would make sense from a strictly logical perspective if these were all the facts but the problem is that the expression "open source" wayyyy predates the OSI. It's been used in the 80s and the 90s on Usenet and in various places, and it simply meant software which was distributed in source code form. (as opposed to binary form) These pieces of software didn't even have a license file attached to them which was implicitly understood as public domain, or "do whatever the fuck you want to do with it".
The OSI simply appropriated the term to a narrower, stricter meaning which grants certain freedoms to the user and takes certain freedoms away from the creator. (Such as the freedom to change your mind in licensing! You can't practically change your mind since these are all non-revocable grants)
Furthermore, I don't really agree with the "leeching off the goodwill of OSS" framing. From a practical perspective as a user, having the source code available is way closer to OSI-licensed software than proprietary, closed-source software. You can view the code of the program, you can usually share your modifications with others (which is technically disallowed but the developers are very unlikely to care as long as you don't resell it), you can fix bugs in it, you can change functionality you don't like.
A less-commonly talked about side effect is that if the source is available, the developers are less incentivised to provide user-hostile features, simply because they know that the users can patch it out relatively easily. There's many cases of rugpulls and forced functionality changes in software nowadays and almost all of it is possible because the user can't simply change the code to change the unwanted behaviour but must put up with it instead.
Naturally, this is a huge advantage compared to closed-source software. And if a company is willing to share its source code with its users, this should be lauded and encouraged! Obviously, the developers want some benefit from sharing the code, and one of the huge benefits is goodwill by customers. If the developers are not incentivised - but more commonly, harassed and mocked - to share their source but disallow competitors to steal it, in most cases, they will simply not share the code at all, leaving you, as a user, with way less freedom in practical terms.
Both you and the company can't really do anything with paper freedoms - you can't really use paper freedoms and the company can't survive if their licensing permits others to profit off their work while discouraging them of doing so. In this view, having more practical freedoms should be encouraged because it helps both the user in having more usable and more modifiable software, and helps the developers not to go bankrupt when Amazon decides to commoditise their offering or something.
EDIT: I conflated "developer" with "company" when writing this post. My apologies. Of course, everything above also applies to natural people who develop software too, not just corporations. I made this mistake because of the subject matter (Microsoft), sorry.