The problem is that your cost model essentially falls apart in three ways:
1) You have to get to/from the ship in the United States. If you don't live in one of the few port cities where repos tie up, it's an added cost factor.
2) Unless a cruise line puts on a very robust transatlantic service, you'll only be able to take the ship one way. You'll have to fly the other way in most cases, and that pretty much eliminates any savings on an all-in basis.
3) Any such service that is put in place would be for leisure travelers exclusively. In the heyday of the liners, they were the only game in town so they got business and leisure passengers. Obviously, nobody will take a ship for a business trip today. And not only would it be exclusively for leisure travelers, but it would be almost entirely for leisure travelers with plenty of time. . .in other words, retirees. Get on any repo and count the people under 62. They'd fit in one lifeboat.
Intangibles: Yes, it's fairly easy to travel around Europe by train, and seasoned travelers in Europe are accustomed to flying into one city and doing an open jaw home from another. With ships, your options to build your own itinerary become cramped by having to return to the port city. Plus, compared to airports, there are relatively few port cities that correspond to places people really want to go for more than a 1-day stop. London, Paris, Rome, Barcelona, Amsterdam come to mind, and relatively few others. There's no way that all of those ports will be served by regular, frequent TA ships. If it were ever to happen, it would probably be London and/or Paris and that's about it.
But I must also add that we've taken a number of repos and have generally had very nice times. Of course, I'm stunned by the number of people who sail to Europe then get right on the bus to the airport to come home. These, I suppose, are the cruisers who are actually frightened of travel unless it's all inside the cocoon of a ship where "everything's taken care of." There's a certain contingent of "cruise only" people on any ship who are in this category and many of them will tell you so. Plus there's another much smaller group of younger people that only get enough time off to take the cruise but not to do anything else. (Of course, they could've flown to Europe and spent their time there, but they didn't. Even though they "went" to the same place, the only place they really went was the ship). Either way, I think it's a real shame. With the half-dozen or so repos we've taken, we've always planned a European itinerary either before or after the sailing.
Finally, this has to be an idea that every single cruise line has considered, and obviously rejected since it isn't happening. Plus, your premise that air fares are through the roof is open to some question. I know that there's always a lot of moaning and groaning on these boards about them, but they're really not that awful.