I actually like this theory a lot. It also explains a major elephant in the room: Section 31. In DS9 they are a shadowy organisation that no one's ever heard of and aren't spoken of openly, whereas apparently 120 years ago they were a whole Waffen-SS with their own ships, advanced technologies and kinky black uniforms.
The original timeline still has DS9 and Voyager, so with a less paranoid early Federation, Section 31 stay in their box working behind the scenes and aren't a big deal. No advanced technologies, and DS9 exists in the TOS world inhabited by Kirk and newfangled tribbles.
The FC-ENT theory is quite long-established and JJ/Disco adds to it. ENT is common to them both but doesn't exist in the prime timeline (at least in the same way).
This is a feature, not a bug. Picard and co return to their original timeline, but they've created a new one that goes on without them. The Picard and the Enterprise of this future are no doubt different from the ones we know and love. This is potentially the future we see in the Kelvin timeline where Spock comes from. It could also be where ST:P is set, but we'll wait and see.I don't dismiss it outright but there's a major hole in the Three Timelines Theory.
If things changed after First Contact, you'd think Picard and the Enterprise crew would've noticed once they get back. So you'd also think either Picard or Temporal Investigations would try to fix whatever was changed. Picard might -- might -- say, "Who's to say which is better?" and let it slide... but TI seems particularly stringent.
The original timeline still has DS9 and Voyager, so with a less paranoid early Federation, Section 31 stay in their box working behind the scenes and aren't a big deal. No advanced technologies, and DS9 exists in the TOS world inhabited by Kirk and newfangled tribbles.
The FC-ENT theory is quite long-established and JJ/Disco adds to it. ENT is common to them both but doesn't exist in the prime timeline (at least in the same way).