The problem with this logic is that events are the only way to reliably get an Elite 5* tank. By making the ONLY use for corns to be the 3rd copy, you put RNG into getting the elite tank. If I want RNG in my Elite 5*, I'll go pull in store.
Even worse is the fact that the shop option for corns is randomized, for even more RNG, which is a huge turn-off in itself.
All in all, IMO they need to make it clear whether events are meant to give a "free" (or almost free) Elite 5* or expect to pay for a perfect 5*. Right now it seems more to pay, except in this case a couple lucky people will get one for free.
And before anyways says that guaranteed Elite 5* is not the big draw of events, I'd like to point out that we get a free 5* every month by default for login bonuses so it's not just getting a 5* that makes events special, it's getting an Elite 5*
Personally, I think the last two tiles should guarantee a corn on the last map of the tile so those who can full clear don't STILL end up with 0 corns for the run. Guaranteeing you'll be at least half way there if you full clear every time seems like it would encourage a lot more spenders (since otherwise it's mostly the whales or the really lucky that will dump cores in)