Not all F2P games fall into the parameters you state above. There are many different F2P models and different games adopt different approaches both in the F2P model and also how the game develops.
What I described is in fact the general scenario. Whether it's a new dungeon or new combat zone, the intent is to get players to buy game currency for real life money, then buy upgraded equipment for the new area. Or are you referring to equipment wearing down quickly such that regular play necessitates continuously buying gold to replenish armor? Even that's still making the game impossible to play without pumping in money over time. UO, however, is based on the flat fee and playing however much or however little one wants. Players are too used to that after all these years.
How long would any F2P game last with no updates? Not very long. F2P for UO is on the premise that EA decided UO had a future, were prepared to go F2P in order to generate players and revenue, and to reinvest revenue into the game to add content and keep the players and the cash coming in.
Thats where it all falls down. EA have no desire to save UO. They will let it die.
Of course a F2P model is based on new content; nobody ever said otherwise. But so is a subscription-based game that releases expansions periodically. Consider this: how long have you been playing UO, and how does that compare to any F2P game? Have you ever played a F2P game that held your interest for even a year, if the game was even designed to stay around that long? For me, I've played UO for 17 years, and any F2P for no more than a few months. I think I may have put as much as $80 into a single F2P game before the marginal value of each additional dollar became effectively zero. By contrast, UO's strength is that for a flat rate, there's as much as I can do, or as little as I want to do.
UO manages to hang on with a dearth of new content because there's still a variety of things for players to do and create, particularly better and better loot, however long the rest of us can remain interested. But you're acting as if F2P would bring in new players, when they'd do just what, do you think? Players will try UO for the first time, or return, because of new content, not even graphics, and certainly not because they can get a month's trial for just $5 (or however little a F2P subscription can be had). Then once new/returned players see how much it adds up, how much they have to keep pumping in, they'd get the same bad tastes in their mouths as existing players, who don't want to spend more per month for each of multiple accounts to maintain the same gameplay. EA may have a temporary surge in UO's revenue, but when people start canceling accounts to cut back, revenue may be below what was before. So it's not a risk worth taking, and EA will keep riding things out. They're unfortunately correct to conclude that that any investment in UO, e.g. hiring a couple more Devs, will not yield a positive return. There's just not the playerbase anymore to support any new salaries. It was EA's inaction, but not today, or in the last couple of years, but for over 10 years. If they had put a firm stop to exploits 10 or 11 years ago, so many wouldn't have quit in frustration for other games. If they had pushed out something like Stygian Abyss in 2007 or 2008, then another one or two significant expansions since, after having stomped out speedhackers and scripters, maybe things would be different. But a long time ago when it mattered, they seemed unwilling to lose any accounts for any reason, short-sightedly ignoring that they lost more accounts overall when legitimate players quit. Now it's to the point that, as much as I hate to admit it, UO may not be profitable enough for EA to keep alive, should they lose all the scripters and multiboxers' accounts.