Chromium is open source, but the supported product where the developer cares about the quality of the experience is Chrome. Android is much the same, where the platform almost all apps and services target requires Google Play.

I can't expect Google to properly address my needs in either of these open source offerings, because the actual users are almost exclusively using the proprietary version.

When Google looks at what the browser needs to do, it can either make the solution proprietary or open source, and then just tell the open source users that it's unavailable unless they switch to Chrome. (See things like browser syncing, or Google Cast support.) But when Firefox implements such features, they have to do so in an open way, which is why you can run your own Firefox Sync Server.