This has been discussed a lot in the past already. I don't think there's anything we can do about it.
AFAIK it only affects devices that have not even updated the Google Play store apps for over two years, or have manually uninstalled updates to system apps (specifically the Android System Webview on Android 5-6). This should be a small minority of devices, and still shrinking.
All you need to do is have Google Play updates enabled and allow it to update - or have done so at any point in the past two years - and it should work fine. We've designed an error message that tells people to do this that ought to appear if it's not supported, with links directly to the store. Why this message still seems to show up for people is still pretty much a mystery. Maybe a lot of people routinely disable updates and uninstall system apps? It seems unlikely...
It's not feasible to backport the C3 runtime to very old browser engines - many of the huge performance increases and major new features rely on reasonably new browser technology. If you need support for much older browser engines, you could keep using the C2 runtime which has lower requirements - but then you'll miss out on all the features and performance improvements.
I think the best solution is to be able to specify a minimum WebView version for your app, so it doesn't appear for people whose devices are too old/not up-to-date enough to run the app. Alternatively it could install any necessary WebView update at the same time as your app. I filed an issue with Google about this earlier this year. I think the best way to make this happen is for everyone to highlight how important it is to them on this Google issue requesting a solution to the problem.