My Dremel’s last me at least few years, and i do not baby them. but i do not run them for long time continuously, or they overheat, especially when ran on 15k+ rpm, nor i cover vent holes with my hand when i work with them, i also use compressed air after each dusty work, i make sure to get as much dust out of the tool as i can. the fan inside the tool sucks air and dust in, as well as anything else that is airborne.
However one has to realize, Dremel’s are tools for delicate, small jobs. If you try to use it to get old grout out of tiled bathroom wall, you will definitely kill it in no time. use the right tool for the job.