speaking as CEO of Element, our business model is really not tied to Synapse being heavyweight at all - just the opposite. We provide Synapse hosting starting at $2/user/month, and so it's critical that running a server (including sysadmin) costs us as little as possible in order to be above water. We're not competing against self-hosters, but catering to folks who aren't sysadmins and so want us to host for them.

And as others have said, Synapse really isn't that heavyweight these days (thanks in part to the performance improvements driven by Element!)