Waiting on a carrier is not the only way to get the most recent version of Android onto your device. There's the option to flash a custom ROM instead.
You can't go that route unless your device is supported, so technically, you may still find yourself waiting. Fortunately CyanogenMod 13 nightlies with Android 6.0 have now arrived for the Galaxy S5 on Sprint, US Cellular, and Vodafone.
These come around the time T-Mobile is pushing Marshmallow to the Galaxy S5. They join the GSM version that works with T-Mobile, which was one of the Galaxy S5 variants already supported.