Completely agree, no developer of an open-source software has explicitly stated that a) his software is open-source and b) that he adheres to OSI's definition.

Most developer just uploaded their project to GitHub and attached a license. If OSI defines some licenses as open-source or not is irrelevant. Legally only the license is binding.

I agree that it doesn’t make sense to let some institution define what open source is.

In the end for open source to be open source it should be publicly accessible.

Anything else should be defined in the license as the maintainer/creator wishes.

Nothing. But their definition is the consensus definition, including being adopted by several governments. There is no alternate meaning with similar support, merely people who don't know what the term means.

As professionals we need to have a meaningful vocabulary. This isn’t Through the Looking Glass.

Again, what gives OSI the authority to define that vocabulary over anybody else? If you want to use "open source as defined by the OSI" more power to you but I suspect most people will continue to use "open source" as "I can find the source code online".

The OSI were the people to come up with the term. Of course they get to decide what they mean.

The only argument against that would be that the term has gained popular usage outside of the open source community and become a generic term. I haven't heard or seen any argument that that should be the case.

Yes - the vocabulary is Open Source, but not open source.