But then the fan absurdity that you can't be grey in the Trek universe without being a MU duplicate turned out to be true, and I lost interest in the character. He ceased to be compelling and much of the misdirect from earlier started not to make sense (why was Lorca so bravely mission orientated in the PU if his goal was to get home and take the Empire?). I don't mind a twist, but I hate being lied to by a writer to make a misdirect work, and that's how I feel Lorca went down. Plus, the stupid burned.
To be honest - he ceased to be an interesting "grey" character at the moment he left a civlian prisoner in an ISIS-inspired torture hell behind, only because that civilian might have ratted out his fellow prisoners to escape torture. AND on top of that, that everybody was okay with his cover story of "he blow up his old ship and killed his entire crew to 'spare' them from Klingon prison".
At that point, all nuance and interesting grey morality was already gone, and he was a straight up evil badguy. And the baffling part was - a lot of viewers didn't even seem to mind. Same how now with Georgiou, who personally ordered genocide, torture and mass murder, hand-picked people she would actually eat, and gloat about all of that in her victims faces. And yet, people are okay with that, because she's funny and snarky and wears a leather-tight catsuit.
That being said - the "fan assertion" that morally complex characters in Trek aren't possible is first of all completely wrong, and second not even a "fan" assertion. Cpt. Jellico from "Chains of Command" was an extremely multi-layered personality. And Starfleet had straight up badguys and moles in it's admirality - "The undiscovered country" anyone?
The assertion that Lorca "could" have been an interesting character without that twist is just patently false - he already was a one-dimensionel caricature before that. People just didn't wanted to see that.