Fast charging has been around for years now and if tons of cell phones became useless because of fast charging, you would've heard about it by now.
Here's some reading material when it comes to battery cycles:
batteries - What is the meaning of a battery's "cycle life?" - Electrical Engineering Stack Exchange
So 80% of the battery capacity remains after 2 years? That's fine by me. Samsung replaces the batteries for $30 a pop on the S6. I'm sure the cost for the S7s are more because of the larger capacity batteries but let's say it's $50; $25/year is fine by me.
Heat is definitely a killer of batteries. That's why Samsung now allows you to turn off fast charging. I turn fast charging off when I'm charging the phone AND using it at the same time for extended periods, like gaming. Why? Because I don't like my hands getting sweaty, not because I want to preserve the battery. Doesn't matter if you use a slow or fast charger. Behavior
Hoverboards? I don't think they use QC technology for fast charging. The manufacturers are most likely just using high wattage chargers that don't even adapt to preserve the battery or reduce the risk of fire.
You guys can take it or leave it. I'll be using fast charging whenever I can because of the convenience. All my chargers are QC 2.0 and it'll stay that way until QC 4.0 comes out and I switch to a phone that supports that technology.
I don't plan on using my phone for years and years to come, I normally swap my phones no less than once every 2 years.