Sounds like a solid plan to me. Maybe the devs would have some balancing concerns with:
{the costs and drop rates} VS {free kobans from event missions (or is your idea replacing those with the free event boosters?) and free kobans from the occasional other daily mission and free kobans from contests top 4 results} VS {still wanting people to actually buy kobans from time to time on top of that}

But there's definitely some potential here.