Yes, the popular mail apps are mostly cloud based (with the notable exception of Boxer, and Google's own Gmail app itself for non-Gmail accounts) and it works well for them.
...
First, Google made life very difficult for non-cloud based apps in Android 6, and it's only a matter of time before everyone runs that version (at least, or some Android 9). Same as now there are very few devices in use running < 4.0.3.
Second, most people don't seem to mind, or are even aware.
Well, I didn't know about that, as I use android kitkat without google account - and never will use one nor any cloud based data apps (email, word, excel, whatever) for data security reason. this is why I never have an account of google, facebook or similar on my devices.
therefore, I am one of those who will continue using your great app
(especially but not only when it got s/mime support
)
edit
btw, what does
...in Google Play, at 5%
mean?