Training is super useful for leveling alts or grinding CP. With ESO+ and a two man group you get + 90-98% XP when killing mobs, thus your time to reach max level is cut in half.
Prosperous is not. It is meant to be a utility trait, not a trait for someone who wants to max their build. So lets make it a trait for those who are just grinding for a new build.
They can choose if they want to get more XP (training) or better/more loot (propserous) while grinding. If you grind CP / max your char you pick training, if you grind armor / weapons / jewelry you pick prosperous.
Its not like people need to grind in their all golden max dps super hero costume. You can do mazzatun or icp or whatever as a duo on normal just fine in some prosperous stuff you find while still grinding for the trait you want to have.