They didn’t sell well even when newly released, but no doubt, fewer over time as they age. However, if they were more popular, Apple would be only too happy to update them more frequently, even yearly like they do MBP and iMac. Apple is plenty interested in sales and the profit that comes along with it.
The Mac Pro is important beyond what its low sales might justify, but assuming the mini gets an update within the next year, it may be for the last time.
Bottom line, you can’t force people to buy something they don’t want. Most buyers want laptops or the all in one iMac.