But tolerance for ups and downs is not a set quality and the gaming environment really influences it. You can run a life-is-cheap campaign where characters are expected to die horribly and maybe the goal becomes who can survive the longest and suffer the most hideous death. That's an experience of a sort. You can run things as easy come easy go and losing a sword or a treasure hoard isn't such a big deal if there's a chance at another one around the corner. Or you can be very very careful not to anger your players and perhaps everyone has a nice even-keeled character development session or perhaps you watch as their tolerance for setbacks grows thinner.
A lot of those are unspoken contracts between the DM and players when the campaign begins. If you expect to have a gritty, bashed, vulnerable campaign, or one that is odly lucky, people don't walk out because they're going to expect that kind of treatment.
However, it takes even an above-par DM to try to run a D&D game with a "life-is-cheap" kind of feel. Most DMs run the game more or less straight. And in that respect, the Rust Monster is out of the realm of expected challenges and totally destroys some of the key aspects of fun in D&D (the acquisition of sweet loot). It disrupts more than it adds. Mearls's revision is to reduce the disruption without removing what it adds. He does so, and thus makes the Rust Monster fit better in a D&D game.
There's tons of ways for a DM to deal with the fighter losing his sword! They don't all have to involve trudging back to town to spend a gp allotment that Negatively Impacts A Character's Wealth. You can have sudden hiccups, plus generous contingency plans... but you do have to have an understanding with your players that their luck may well turn for the better if they hang in there.
It's still a major disruption. What if the PC's manage to get lucky and their equipment isn't damaged? Suddenly that windfall is potentially game-breaking. What if they get unlucky and get damaged far beyond what that windfall was to compensate for? Suddenly, they're giving up on the game and calling it too hard.
Major disruptions have a big chance to make people angry, at either their lack of power (And slavery to DM's whims), or at the amount of work that needs to go into "fixing the problem." As a DM who doesn't want to spend more work than nessecary on a fun night's game, the revised rust monster is a whole lot better than the one that came before.
Yeah, it still has issues. It's inelegant (it routes around the way the system already handles item damage because it thinks it can do it better). The "10 minute recovery" thing makes little sense (though it could still be easily removed with a Craft check or Mending spell). But the issue is no longer "If I include this monster in an encounter, I will have to prepare the next month's adventures around the PC's rusted items."