They try super-hard to do so every time there's a change on the account (probably 90%+ were lost when the "free phone" came with a new contract), and they've raised prices (+$20/line). They also instituted the 200GB (maybe 100GB) limit.
The amount of hell they would receive for doing so, T-Mobile got a large amount of flak for transitioning legacy customers to Simple Choice plans. Instead of incurring the PR nightmare that would be, Verizon is just making these plans unappealing with soft caps (they did this to force people off unlimited mobile broadband plans, the number of customers I had yell at me when I worked for a VZW call center because their internet connection was severly throttled was mind-boggling) and by refusing to give subsidized upgrades without changing to a newer plan.
http://www.droid-life.com/2015/10/08/verizons-20-unlimited-p...
https://www.washingtonpost.com/news/the-switch/wp/2017/01/11...
http://www.droid-life.com/2016/07/26/fyi-verizon-unlimited-d...