Gold is extremely hard to get. You only get a few hundred to maybe ( if lucky and you get teamed with a heavy P2W ) a few thousand for Co-Op battles. You need thousands and tens of thousands for upgrading cards and buying cards to upgrade. Upgrading Legendary cards is near impossible for F2P. Obtaining Legendary cards near impossible. Massively increase gold from Co-Op and maybe more people will do it. But atm, very little reward for what could be up to 30mins!