Every exercise app I ever used is all like: You have achieved your goal. Now set new goal and push yourself harder.
Why can’t it ever be: You’ve achieved your goal. Well done. If you ever want to go further, you can change the settings, but for now - enjoy it!
I personally liked the PAI approach from the Mi Band where you get points for various activities, depending on the heart rate during the activity. You loose the points after a week - motivating you to continue and also the points are harder to get the better you get at the activity, because of the lower heart rate. I know, it’s all smoke and mirrors, but I I liked that approach for the same reasons you mentioned.