A 739ER can't do the flight westbound nonstop with any decent payload. For a 738 to make it westbound all the stars have to align, meaning no storms can be in the flight path, the flight plan can't deviate into Canadian airspace, the winds aloft have to corporate. Anything that increases the fuel burn on the 738 and UA ends up with a plan or in many cases (in the past) an unplanned fuel stop.
You've posted this a few times, and I disagree with what you're saying. I don't have the NOC view like you have, but from a pilot's view, the 737NG is a bit more capable than this forum gives it credit for. And UA has done a great job of planning the aircraft around the limits it has to maximize its potential.
1. 739ERs flew EWR-SFO, EWR-LAX, etc prior to the move of PS service from JFK to EWR. They did fine. There might have been a few diversions or planned fuel stops, but it was a lot less than you make it sound. I personally have never had one. I don't even remember blocked seats. The only time I noticed a weight restriction was on a straight 900 going eastbound, and for that airplane to get into that routing meant that we had to be short on jets that day.
1a. In fairness, I have no knowledge of the amount of cargo the airplane took or did not take. I'd imagine the 752 did better here.
1b. I never flew BOS-SFO enough to form an opinion. I would imagine this is fairly challenging for the aircraft in certain situations.
2. Flight planning software accounts for winds and plans the least cost route, even if the flight must cover more ground to take advantage of a lesser headwind. It is somewhat common to see Newark transcon departures go through Canadian airspace if the winds justify that routing. The flight planning software is even smart enough to calculate the cost of Canadian overflight and route accordingly.
3. To this day the 9ER flies similar transcon routes ex-PS: EWR-SAN, EWR-SMF, IAD-SFO, IAD-LAX. Also, ORD-ANC, while only operated in the summer, was UA's longest NG route, I believe. It was typically an -800. Again, I fly these routes routinely and have never seen a planned fuel stop.
3a. ORD-ANC had some special rules about a go-no go decision to be made enroute, similar to a redispatch scenario, however the flight is normally dispatched. This was mostly because of the lack of viable alternate airports closer to ANC.
Where the NG fleet struggles is on the Hawaii routes. Even though SFO-HNL is less distance than EWR-SFO, the HNL flight must carry about 5000 lb extra gas for ETOPS fuel. The airplane literally did not have a big enough fuel tank to carry all the gas during the windy season-- typically it was fuel volume limited and not weight limited. So, what the NOC would need to do is reduce the weight of the airplane (block seats, hold cargo), to reduce the fuel burn. The MAX, whenever it comes back, fixes that problem because while it has the same fuel capacity, the engines burn about 10-15% less fuel (which is conveniently right around the magic 5000 lb number for ETOPS add fuel).
I have no doubt in my mind that the 752 can easily be replaced on transcons because we did it before. In a high-RASM environment, like what we just came out with, having the lie flat seats was worth the fuel cost penalty on the 752, but with revenue taking years to recover, having a lower cost fleet flying the routes is probably the smarter move. Obviously, this is just my opinion, but it seems this is the direction we are heading.