When I joined Strava, it was very motivating to get achievements as I improved. Due to an injury and age, I'll probably never run as fast as I did a few years ago, so I never get see those achievements as I slowly get better. While getting an achievement for beating a lifetime record is still great, it would also be great if there were also achievements based on activity over a user specified period. For me that would be since I returned to running in November after my injury. For someone else, it might be when they shift their focus from short runs to long runs or for runners that take the winter off an annual thing.
Ideally, you'd be able to pick a date in past to compare to rather than a hard reset - a hard reset will make every run achievement for a while which makes them meaningless.