I think there will be no penalty for using Cards of a lower Level and it is possible for a lower League player to encounter Max Level Cards. This would lead to the stronger player move into higher Leagues faster. I have been in situations where I couldn't use the best Cards I had because of the limitations that kept me in lower Leagues for a longer time.
Another good thing to come out of this change is that a completely new player that buy or rent few Max Level Cards can easily climb up the ranks very fast. There will be no reason for them to limit themselves to low Level Cards. Prices for Cards should improve after the change.
I agree on everything you said. I expect an increase in rentals for everything. I just hope they don't go overboard with it. I still remember the level 4 CL summoners at 40+ DEC per day. Made me stop renting completely once I realized how much I was spending in all of my cards per week. (And I never owned them coz it's renting.)
I could probably buy at least a max level legendary for CL if I saved all those DEC.