As with FlyingTin, I often make my booking decisions (and try to book early) based on which specific seats are available, especially on longer flights. Doesn't seem to make a difference to me whether you want E+ bulkhead because you want to use your laptop, have long legs, be close (or away from) the lav, sleep, whatever. While there may be good reasons for the carrier to deny you the seat you reserved, this doesn't sound like one.
I can't imagine asking a pax to swap to an inferior seat, unless I'm willing to offer them a bit of cash for their inconvenience. Many comments here on the board over the years suggest that we're generally happy to consider seating alternatives when politely asked, and rarely will move gracefully when forced, especially for what seems to be specious (or manipulative - trying to get an extra seat) reasons.
In this case the couple was given an extra seat because they were "uncomfortable" sharing with the OP. Frankly I am almost always "uncomfortable" when someone sits next to me on crowded planes. I'm
"uncomfortable" sitting next to someone who is obese, or uses a lot of
fragrance, or flops all over me when they sleep or...
But back to the OP's question, it seems to me that as long as the crew have
the "interfering with crew" trump card, it is hard to fight back these days. Maybe OP would have had a chance if the FA was on his side; don't know what clout the GA has with the cockpit. But perhaps digging in your heels a bit more would make them hassle someone else and try to keep you up front in E+