That is incorrect. We did it before during an event 2 or 3 years ago whereby we needed to upgrade to higher stages too.
Or during the event when we needed to transmute a thing and then open the box to get the event currency, after the event we could still open the box (not the box giving items only like last Christmas event).
After the event, we cannot get any extra items to benefit us more beyond the event period, I agree.
BUT, fodder items or intermediate items as part of the event shall still be useable or redeemable, because event redemption period IS NOT OVER YET.