Why don't they adopt click-to-play trick to get the best from both worlds?
This is part of the reason Flash was blacklisted from iOS: features have to wait on Adobe’s schedule to be enabled, instead of being available out-of-box.
Google isn't holding back on major OS updates because Adobe isn't ready.
And it does. And 3rd party plugins that Apple has no control over reduce the quality of that experience, increase support costs, and generally add instability.