I was a contributor to FOSS until I had to make a choice to step back from excessive keyboard use due to RSI. I love and embrace open source software and the ideals behind it, but I believe everyone should be given their own freedoms.
The plugin boundary should be exactly that, a boundary between the primary app and any created plugins. Why do you feel entitled to force a license decision on a third party's work? Yeah, my work might not function without your code, but your code is your code and my plugin is mine. If I choose to write a utility that enhances your code in ways I want, is that not the benefit and flexibility that FOSS clamors about? Hell, what if someone else adopts a similar API and I choose to take my plugin that direction? That's my choice to make, but forcing GPL on my code may now be hampering my freedoms.
I hear the argument of "if only software X was open source, then I could fix this bug here or add this feature there". I'm right there with you on code that is in the mainline and lives inside the source, but if I'm writing a plugin, that's my work and I own that, not you.
If I choose to make use of a proprietary library because it makes sense to me, like for example CAD solvers where the FOSS alternatives aren't there yet, that's my decision for my plugin. All your GPL license does is infect my work.
That's why I don't use GPL, and why I agree with others that the end result of many GPL licenses is to simply reduce the overall amount of GPL code in the world, as people avoid it out of concern of risk.