A background blur is very useful when implemented as a blurred background via coding in websites and apps. Imagine a tab bar at the bottom of an app (or a navbar at the top of a website) that live blurs the content that scrolls behind it. That's how this feature is designed to be used (and that works). It's used in many iOS and Mac apps for example.
It's not really meant for creating vector graphics (but keep in mind it can be used to create pixel-based graphics). Plugins can't help because they can't change the coding standards of websites or apps. I understand where you're coming from, but how you're using the background blur is not how it's intended to be used. It's a cool way you're using it and you're not wrong for wanting it to work! But you'll have to use pixel-based graphics if you want that effect in a graphic.
There is a body that decides coding standards for websites, but considering that CSS can already do this already (which is different than SVG) they either know about it and may already have a plan to implement it (who knows when), or it might not be something they plan on adding to SVG). Either way it's out of Adobe's control and web coding standards can be slow to change.
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Adobe Certified Expert & Instructor at Noble Desktop | Web Developer, Designer, InDesign Scriptor