It only even exists because of the hipster mentality to "be different" just for the sake of being different with no real other point.
That's quite the revisionist history you have there, though I do agree with much of the rest of what you've said.
In truth, both languages saw their first releases in 1983 (though they had both been under development for a few years prior), both languages were being actively developed in parallel at that same time, and Objective-C was selected as the basis for NeXTSTEP back in the mid-80s, before the meteoric rise of C++ had yet begun in earnest. Given Apple's financial state in the late '90s, their desperate need for a new OS, and their past failure at building a new Mac OS themselves, they were forced into a position where they needed to purchase an OS from someone else. As such, their decision to purchase NeXT and use NeXTSTEP as the basis for what would become both OS X and iOS was born more out of desperation and an acute awareness of their precarious financial situation than out of a hipster mentality to "be different".
That said, I completely agree with your opinion regarding which language is the better one to learn. For all intents and purposes, Objective-C really is just single-vendor language, as you pointed out, and it's slowly being obsolesced by that vendor in favor of a different language, meaning there's an indefinite expiration date on its life. In contrast, C++ continues to be actively developed, and while it may be getting overshadowed a bit in more recent years, it's still strong and will doubtless still be alive and kicking for several decades to come.