I realize your point, but no grease means it runs hotter, at these speeds air will not cool it as it runs in an enclosed space. The seal makes it impossible for air to pass through even if it could. No air flow = no cooling.

Normal heat from bearings is taken out through the contact of the inner and outer races with the shaft and pulley. It is the surfaces of the ball and races that see the most heat and that is were the failure starts when excessive heat is generated. Oil flow cools the best but this case has to rely on grease. Lubricants keep friction down and oil is the best. The whole bearing does not see the extreme heat, just the surface. Heat and excess speed causes the lubrication to break down.

I apologise for my wordiness but it requires a reasonable explanation.

PS. Stainless steel is a poor conductor of heat and that is one of its drawbacks, the heat stays on the surface longer, allowing the temp. to rise faster.