That is a really bad suggestion and not how the rating system in the app store should be used.
Garmin will not use those ratings for the development. Instead use the suggested way to send your ideas. If enough people ask for it then it might come. https://www.garmin.com/forms/ideas/
Custom workouts is probably hard to add since so much relies in the system on the predefined exercises and how you build workouts. What probably would be easy is to add a few workouts named "Custom workout 1", "Custom workout 2" and so on that could be used. Even an alias system might be easy to add where all workouts could get an alias defined by the user. That would help a lot and then you could "rename" the already existing workouts and the custom workouts.