Interesting points. However, I think the reasons are much simpler: why wouldn't you want accelerated CSS in your browser, including cross-origin iframes?
Browsers have been making use of GPUs for a long time. It's faster than the CPU and saves battery
Yes, between the device with hardware accelerated rendering and the device without, the user is likely going to prefer the former because software implementations are much slower and much more power-hungry. Thankfully SVG/CSS filters aren't used a ton on the web but usage has probably crawled up over time - I know many websites now use CSS blur filters.
Browsers have been making use of GPUs for a long time. It's faster than the CPU and saves battery