It's odd that Mort didn't rule out Superboy and others visiting from the 20th century. They must have had adventures when only the 30th century members were available.
Obviously, the whole tax business was just a fig leaf to provide some kind of in-story motivation for the editorial mandate that Superboy be written out of the LSH. I'd rather have seen him suffering some ill-effects from all the memory tampering that had been going on to keep him from recalling anything about his own future, so the Legion would have no choice but to make him stay away (undoubtedly with more mind-tampering, because that trick never backfires). The tax thing was pretty weak, if only because it wouldn't have been hard to produce proof that Kal-El was legally dead as of the 30th century. At least, I think he was--I know that there was at least one story in which the death of Supergirl was presented as an historical fact in her own feature (a history book from the future found its way into the Stanhope College Library), but I don't recall any direct mention of Superman actually dying, even in the 30th Century Superman stories. His biggest future revelation was always who he'd marry.