Good points. I joined in 2016, so not a "legacy" member, but when I add it up I have paid the best part of £2.5k over the last 9 years and I'd hate to lose the right to vote. As @Brazilian says, you never know what's round the corner, finances wise.
I think your "lifetime membership" idea has merit, although the initial qualifying amount might need to be higher to reflect the passage of time since 2015, and it would probably need a pre-agreed mechanism for periodic resetting to compensate for future inflation.