The fundamental flaw in your argument is that systemd-resolved is not a resolving proxy DNS server. Many other proxy DNS server softwares indeed are switchable between resolving and forwarding modes, some even doing both at once, but systemd-resolved only provides forwarding proxy DNS service. There is no resolving mode to default to.

Which is in fact the core of the problem. It's actually a fairly rare kind of proxy DNS server software, one for which there isn't really a sensible universal default for the unconfigured case.