Compare that to an average lifespan of 10-15 years for a feral horse.
Feral horses get all the same diseases and physical ailments that plague domestic horses including founder, uneven wearing of their teeth, colic, etc.
The difference is that feral horses don't have humans to look after them, so either die of their ailments or get taken out by predators. That's how the myth of the feral horse being so much more 'hardy' and 'adaptable' has come about.
It's also how the myth of them having such great feet got started, and a whole slew of charlatans and fanatics touting the 'mustang barefoot trim' way of life.
It's not that the feral horses don't ever have bad feet, it's that they don't live very long when their feet do go bad.