In 2020 when Muse is discontinued, will I be able to edit my site using in-browser editing?
Yes. Technically it depends on browser compatibility and your system compatibily or clients system- and browser compatibility.
This will be one year or several years. As of this reading nobody knows, when this will be.
I recommend to be prepared for worst case scenario and tell clients in time, that there might be some changes necessary.
This will cost some money.
Check out from now on, for another app to create websites without code or learn code, if you prefer that.
Only a possible indication for the life of in-browser editing, but when it was discontinued in Dreamweaver, (I think it was in CS5) it stopped working within 6months, and even though 3rd party developers started to offer the feature, those stopped working after about 2years.
Just as a note though - in-browser editing features are part of a W3C spec, so in theory it could work for many years, the problem with the feature though, is that the hosting server must support the required 'additional' security features, which is the main problem for support.