Not entirely the case. Some Micro USB can currently charge just as fast as USB-C. From what I gather. QC 3.0 is the current generation standard found on many Micro USB. This is considered Fast Charge. QC 4.0 is the next generation that will be coming soon(ish) and only going to work on certain devices. Unconfirmed/unknown if the S8/8+ will be included. It meets the requirements, but that's up to Sammy I guess. Chances are it will. Anyway. 4.0 will be only on USB-C and can be up to 20% faster than current 3.0 fast charge. Problem is waiting for 4.0 tech to arrive and manufactures to take advantage and create products with it. That could take some time. I'd say your best bet is to settle for a 3.0 as it can be months (if not longer) to find a good 4.0/USB-C mount you like on the market.
Actually... nope. It IS pretty much confirmed that the S8 line does NOT support QC4 (even though Qualcomm's chipset supports it) and furthermore, it doesn't even support QC3 (both due to royalties since Qualcomm holds the patents). The S8, just as the S7 line and the Note 7 before them, it's compatible with up-to QC 2. Samsung's Fast Adaptive charger, however, is supposedly just as fast as QC3 (but your phone will charge with a QC2 charger or compatible).
As for QC4 being USB-C only, that's true for wired connections. For wireless, it doesn't really matter as even fast wireless cannot output the needed current levels as of yet, so power from microUSB chargers are still more than enough to power the wireless transmitters.
Also remember, Fast Wireless is not as fast as Fast Wired charging (it's actually closer to just 'normal' 2A wired charging speeds). So as long as you have a 2A charger powering it (most modern microUSB ones) you'll be fine on a wireless charger.