You'd be very surprised at the people willing to toss money into stuff like this. They've proven people buy into this stuff time and time again in their other games. World of Warcraft charges exorbitant prices for various things like $25 per toon server transfers, $10-20 for little cosmetic companions that follow you around, $15+ cosmetic gear, $20 tokens people sell for gold and who knows what else is to come
Heroes of the Storm has $10-15 skins and charges $5-10 per hero. Hearthstone has people spending literal thousands on virtual playing cards to finish decks.
Overwatch won't be in any different when it comes to people throwing money at what ever Blizzard offers. Not everyone will do it, but there will always be people who buy into this stuff which means they have no incentive to offer these things for free beyond the finite amount of lootboxes you get from leveling because why do it when there's microtransaction 'whales' out there willing to throw hundreds into virtual item boxes?