It's not about outright bearing failures... it's about /heat/ (which can lead to failure--but "good enough" bearings will last "okayish").
Bearing friction causes heat--and it has to go somewhere... better/lower/non-friction seals, better grease (or an outright oil bath like Mike is experimenting with) can significantly reduce the heat produced (and thus also make it more responsive). Simply choosing the wrong grease can also result in early bearing failure--if the grease doesn't have a high enough speed factor rating, then at high enough RPM it can literally get wiped away and result in metal on metal contact--lots of heat and possibly also physically damage the race.
A full-contact seal accounts for as much as 60% of a bearing's friction--so you can imagine what going to a non-contact seal can do.