That number is not growing as younger people move away from pay TV models, they aren't embracing OTA, they are embracing streaming.
As to the ATSC3.0 stuff, well adapters will come out as they always do if/when it becomes an actual issue.
http://www.thesimpledollar.com/cord-cutters-why-millennials-are-saying-goodbye-to-cable-tv/
"Why millenials are saying goodbye to cable TV"
"ON MY TV
As mentioned, some cable companies are reaching out to cord cutters, offering packages that include high-speed Internet and TV via a digital antenna. Now, if you're a millennial, you might be thinking, "Why would I need an antenna in 2016?" Here's why: The four major networks (CBS, FOX, NBC, and ABC), PBS, and the CW, as well as many smaller networks, broadcast digitally over the air in most markets, meaning you can watch each channel with that antenna — for free. If you just want to watch popular shows like "NCIS" or "Brooklyn Nine-Nine" and, say, the local news or Sunday's football game, that antenna might be all you need. Just make sure you have the right one. AntennaWeb's handy tool can help you figure out how close you are to the nearest transmitter, what type of antenna you'll need, and how many channels you'll receive. This will be the cheapest option for cord cutters, but it'll also yield the least content. You're basically living in the '50s, at the mercy of network programming. But, like we said, it's free."