I think this would be a good idea especially if it counted to 999 or 9,999 and couldn't be 'fiddled with'.
Obviously once the log is written the name/number can be changed but for any log being produced the 'correct' number would, I think, be a useful counter for buyers of second hand drones.
Would the programming be difficult?

This would be a bad way to judge how much usage a drone has since a 35 minute flight would carry just as much weight as a 20 second flight. A better way would be similar to how batteries keep track of how many cycles have been put on it. More importantly, DJI isn't going to do either one of them. There's no incentive in it for them. If anything, they want to discourage the secondhand drone market. They don't make any money when you sell a used drone to someone. When you do this you actually just took away a potential sale from them. We all know how greedy DJI is and that they wouldn't implement something like this.

Used drone buyers already have too much to obsess over...it sometimes feel like they are not buying drones, but rather think they are battery connoisseurs.  If you want a fresh battery, buy it.  if you want a used drone, then there are plenty.  if you want a new drone, go to DJI's store.  : )