The cost of the adapters is only part of it. Adapters are inconvenient, and IMO the reduced thickness isn't a good tradeoff for that extra inconvenience. (It has been in the past - losing the optical drive and ethernet port was absolutely worth it to me for a lighter & thinner machine.) Not to mention you only have one port, so even if all your USB devices use type C, you'll still need an adapter just to charge your laptop and use a USB device at the same time.