The problem is, even if there are only 5% of all app-downloads which have the white-screen error, it's very bad for your game.
It's not actually clear that this is the same problem as described here. Normally an error message should appear telling people to update. Old devices with out-of-date software often also have buggy GPU drivers, which can cause problems like only displaying a blank screen. This type of problem also happens a lot and is unfortunately inherent to the Android ecosystem, affecting any kind of app using OpenGL.
As far as I can tell our "not supported" screen is working - you can see it in old browsers like Internet Explorer where it appears correctly, and we've not had any other reports of the screen not appearing (indeed any questions usually refer to the text of the message and asking more about what it means, suggesting it showed OK). So I suspect that actually there is something else going on - or a combination of both issues, perhaps - so solving the webview issue may well not actually fix the blank screen problem, nor completely stop 1-star reviews (which lots of people seem to submit arbitrarily, even for apps like Construct 3 itself). It's important to fully diagnose what's really going on before jumping to a conclusion, to avoid being sent on a wild goose chase that may not actually help.