... sure they will lose players that were here just for the rewards. If you achieved Champion League with Diamond level summoners (LEVEL 7 and 8 rare), then you shouldn't worry... If you achieve Champion League with Gold or Silver level summoners, then you shouldn't have been in the Champion League. They have ALWAYS had the leagues broken down by summoner size, and its only due to people exploiting the bracket creep by botting that helped everyone get out of the league they were intended to be in.
I don't know the size of your deck, but some people have argued that everyone should be in the Champion Level league since they got there by the system in place. Of course that is silly if the system in place was broken. I'm glad they are fixing the system to give everyone a more pleasurable experience. I think there will be some people that leave as you suggested, but there are also many more players that are happy about this change. The obvious flaws of low level decks getting to the higher levels made the game not sustainable. It had to be fixed and its about time they did it.
RE: New Reward Cards & Collection Power Update