I kind of like the idea.
Although I'd ballpark $50 and $25 as more reasonable figures. It is such a small amount to pay for potentially rewarding benefits, that most people who care about their miles would pay for it.
Remember, most passengers -even elite customers- fly an airline based on a mix of schedule and price (if the airline doesn't fly there, it has no schedule... if the airline has awful times, it has a bad schedule...).
Rarely do people care about what hubs they connect through, and they will go out of their geographical way if it saves them time, money, or both.
The number of pax -even those truly loyalist elites who stick by United in its small markets- who go OUT of their way to fly United -costing them time, money, or both- is very small. This segment is very rare. And though you might alienate them with a fee, well if they're crazy enough to stick with United where it doesn't make the most logical sense anyhow, they might just be crazy enough to pay the fee.
Also, for the people who really value United's mileage program, much of them value it for the destination possibilities it provides. United has, after all, the most far-flung international network. True Freqent Flyers will not ditch because of a fee since no other US airline can offer as easy mileage access to destinations such as Australia.
I have been working as a gate agent for the better part of the last year. Let me tell you, most Premiers and above don't choose United, but out of necessity, corporate discount, or the overall opportunities to earn/redeem miles and fly in United's worldwide route network.
Most leisure pax (90%) wouldn't pay a significantly higher fare to fly United just because of Mileage Plus. To most that I've encountered, earning miles is just a "freebie" and not any sign of loyalty... they only fly United -again- because of a mix of price and schedule. To the leisure travelers whom the miles REALLY matter (those saving up for Australia), I think they would chip in even though over a period of 5 years, the cost of membership in M+ would be $125 to $250. And for the ones who don't really value their miles that much (view it only as freebies)... by far most will continue to fly United based on a mix of price/schedule.
Another way to look at this would be to charge "award redemption" fees, which could be more variable on the type of award ticketed.
no wire hangers!