United seems to like to hang onto extremely old airplanes even as the number of these disruptions mount. We can argue how statistically they're safer etc but these events are extremely unsettling and disruptive for passengers and frankly it's lucky no one's been killed yet. One of these planes dropped a wheel on a parked car at SFO last year.

It's not hard to notice there are other major airlines that generally maintain newer widebody fleets.