Yes. That's the very idea of GPL. One's either accepting it and gets what it grants, or not.
> BSD/MIT automatically implies open source.
Ok, I simplified the wording and the meaning was lost. Let me fix it. I meant, I got a binary but that binary was produced from a BSD/MIT/Apache/whatever-licensed source code. If that source isn't available to me (like on the almost every single Android phone we have out there), I'm basically powerless.
> Proprietary software can be open source.
This is certainly wrong. Unless you're not using OSI definition of open source, but something of your own. Or if we mean something different under the term "proprietary".
> Case in point: GPL/share-alike adoption has been in steady decline.
Not surprising. GPL had lost, you're totally right on this.
It was predicted long ago - as most users don't possess any engineering skills, they don't exercise their freedoms to access the source code, modify it and distribute the modifications. The freedom to distribute the original software lost it to the unauthorized distribution ("piracy") and freemium-type software. Users don't care (actually IIRC it was shown that they tend to cope with whatever inconvenience they find, rather than try to fix it - that's also how privacy wars were lost), and developers find it that proprietary software has a well-known business models to bring profits, while free software requires to tread carefully.
Yet, it's wrong that "all it does is stop devs from using it". The word "all" is surely incorrect. It does stop some devs from using it - two groups, actully: a) those who want to develop proprietary software and b) those who just can't stand it for any personal reasons. But that's not all it does - I've mentioned what it also does above.