So we see this feedback a lot and here's my thoughts on it.
If it's untradeable then you only get them on your alt unless you pause all main-game progress which a lot of people won't like. Most people are going to like the rewards being tradeable so they can do this. Ironman accounts are hard done by this, there's no way around it though we don't have the tech to let you log in to more than one world at once.
Also, there are benefits to being tradeable. They will be rather expensive when the league ends and everyone gets them and people get to feel like they earned something which helps their main account progress.
And finally, we do want a way to show off your progress in the league, this is why the trophies are untradeable. Each league the logo will change so if you want that prestigious dragon trophy, better play and get it!
Hopefully this means the rewards have something for everyone :)