what i've personally experienced/witnessed regarding retirees opting into the advantage plans is in large part b/c they either don't have retiree health benefits from a private employer OR (VERY COMMON in recent years)-they retired from public service jobs that 'guaranteed' retiree health benefits but those employers no longer offer them to new hires and are doing their best to get those still eligible to opt out by choosing the most obscenely expensive plans that often have retiree premiums at obscene costs (4 figures a month for a single retiree w/no spouse or dependant covered) AND (based on evidence uncovered in a healthcare benefit lawsuit between retirees and my former government employer) are chosen by the employers to further alienate usage by limiting or entirely eliminating reasonable usage/coverage (in my case the only option was a single ppo but they had no preferred providers w/the insurance company pointing out that they are not obligated to recruit/seek them out but simply offer the opportunity for providers to sign on

).
i know it's not common anymore for employers (private) to offer retiree health insurance but there was a time when straight vanilla medicare could cover a person fairly well. with the advent of costco,
walmart and other lower cost resources for glasses it was'nt a huge burden to be without coverage, dental? honestly i've never had a plan private or public sector that paid a great deal so the average checkups/cleanings/xrays were/are not a big financial burden. where i see the big uptick in cost (average person/average senior meds) is perscriptions. i look at what meds cost today vs. when my mom passed in 2011 and it's insane. vs. what it was costing her with just vanilla medicare in the early 2000's it's out of control.
i opted into the advantage plan b/c i needed to drop my retiree coverage. my biggest consideration was perscriptions. the advantage plan i have offers very good coverage and still allow brick and mortar pharmacy usage-which i found that other perscription coverage plans did not (only mail order unless documented, by their criteria-urgent/emergency). if i was given the option of simply having 'vanilla medicare' and it offered just the perscription coverage i receive now-i would gladly continue to pay the additional premium i'm paying now for my advantage plan and forego all the extras they offer but largly go unused on my part.