If you practice Genji in QP, you will 90% fight people playing heroes they don’t usually play. You duel Tracers and Doomfists who don’t main their hero. You go solo hunt snipers and healers in ways that gets you killed in ranked games. You mindlessly use your blades, and never learn to pay attention to support ults.
List goes on forever, it’s just a bad place to practice anything except rookie mechanics like not accidentally dashing when you were supposed to deflect.
EDIT: Also, about this:
If they take their rank so seriously in gold, they will practice enough to climb out instead of complaining to their Genji.