Well, yes. You have to remember, for the actors, it's just a job. I'm not trying to imply they're lazy or don't give a damn, indeed the opposite is quite the case if they are indeed making convention appearances. But at the same time, a job is just a means of making a living, and they shouldn't be expected to keep picture perfect details in the memories decades after the show endeds
I accept the point that it is just a job for them and that they shouldn't be expected to remember a lot based on having done that job a long time ago.
My counterpoint is:
Since money is changing hands, that seems to complicate the point that you've made. The actors are not appearing at conventions out of the goodness of their hearts, as charitable favors to their fans. Rather, they are appearing at conventions because their fans are paying them good money in order to give them new jobs. Yet the actors don't seem to be doing any new work in exchange for their new pay...other than merely showing up and talking about random things that are mostly irrelevant to Star Trek.
Could one not then argue that the fans are not receiving a good value in exchange for the dollars that they pay in order to hire the actors to show up - and presumably - talk about Star Trek?
Could one not then argue that the actors should do the new work of watching their shows (in order to prepare themselves to discuss those shows at conventions) as a fair exchange for the new pay that their fans are giving to them?