I've used high quality air cooled spindles for years with no issues, running 8-10 hours a day, 6 days a week. HSD spindles with electric fans for over 10 years at a previous job.
At my current job, we have a 10HP spindle that's cooled by the dust collection airflow. It's been running full time for over 8 years now.

I own a couple air cooled chinese spindles, but haven't used them yet to see how long they'll last.