It took me months to find housing spot in UO.What if those commanders that left the game for whatever reason, come back only to find their discoveries gone? Have you ever left a game, and then come back years later? I know it happened to me in WoW where I played through the first 3 expansions, left for 3 years, then came back. If they had deleted my main, and all the achievements, including server first kills I'd never even considered going back. That whole argument you make about "meeting them in game" is bogus. First to discover is first. Whether you can or not meet them in game is 100% irrelevant to that fact.
If I left for more than 10 days it would disappear and someone else would claim it.
Games have a largely transient player base and making room for new players makes sense to me.
That said, I think it would be weird to leave a game for a couple of years and then come back expecting that one's fame and fortune had remained the same.