Interesting thread but…surely there comes a point when, if your memory isn't up to it then its time to step aside for younger singers? On the other hand, the only thing that strengthens memory is memorising. Committing stuff to memory isn't simply a case of remembering words but is also connected with patterns and mnemonics that make remembering easier. The odd thing about the process is the more you have to remember, the easier it is. Vic is absolutely right, people have a duty to their audience.
Another side of the coin is this: We have a younger son who sings strongly and in tune. He won't go near a folk club because he doesn't trust people's applause. He remarked, 'If they clap for crap as loudly as they clap for me how do I know if I did ok?' His view is that it isn't worth performing for people who aren't critical. On another level he'd rather be booed than feel patronised.