Why would someone want to stay with Plan F-HD even if the plan is closed to younger members? Because 1. Plan G will now be a guaranteed issue plan so rates will go up faster from now on. 2. From the rates I've seen so far for Plan G-HD on a senior insurance forum they are considerably more than Plan F-HD. 3. In order for Plan F-HD to be adversely impacted by any changes, the average person on the plan will have to have almost $12,000 in annual medical bills year in and year out. The current average for all of Medicare is $800 according to my Medigap insurance broker, although I see a slightly higher average on kff.org. 4. Plan F-HD definitely saves someone money when they are healthy and even when someone is in their 80s, the Plan F-HD rate plus deductible will likely be cheaper than the other rates.
Here's a good article on why a high deductible plan may be good for some people: https://www.journalofaccountancy.com/newsletters/2018/mar/high-deductible-medigap-plan.html