That's why I call TLS keys "ephemeral" under this threat model.
The goal of anti-E2E legislation isn't to be able to MITM a conversation - again, government agencies can already set that up with the current protocols fairly easily. The goal of the legislation is to make it so that, "with the correct keys that only the good guys have", you can decrypt any past message you want that was already sent using the messaging system, without needing access to either device.
If the governments only settled with an "active tap system" that works like a MITM for e2e encrypted channels, we wouldn't be having this discussion or we wouldn't be talking about new regulations. Because again, that is already possible, and governments are already doing it.