I think it's just provided less incentive for developers to handle 'unhappy paths' within their app — there are still plenty of ways that you'd want a webapp to

I think it's just provided less incentive for developers to handle 'unhappy paths' within their app — there are still plenty of ways that you'd want a webapp to handle unexpected closure (e.g. user accidentally closing a tab, browser crashing, network connection dropping before save takes place). If an app is handling these states fluidly they will likely get the 'accidental back' problem solved for free.