Verizon doesn’t do that luckily, I believe they suspended new line activate with 5S however for existing lines they are not disable your line like AT&T yet.
I also don't really understand what you mean by "slowly force everyone ASAP". That's sort of a self-contradictory phrase.
Almost everything supports GSM, so almost everything can manage, and then nobody needs to throw out their 2G only (or 2G/3G) car junk or power meters or who knows what else. Yeah, it's going to be slow, but whatever; people who want something better can upgrade in most cases, and the 2G usage can't be that high anymore. Carriers could even gasp cooperate and allow roaming on 2g, so you didn't need three of four different networks each spending a sliver of spectrum on it.
4G: 600 MHz, 700 MHz, 1700/2100 MHz, 2300 MHz, and 2500 MHz, bands.
5G: 24 GHz up to 54 GHz.
Sure, it's an eventuality. But in the near term, is 6g really going to use 900MHz?