The point is not the new content as the good IL servers don't have regular content, they actually have custom "new" one.
L2 is the same engine since 2003 and it will continue like this till the end of time.
The point is that when a random admin owner tries to open a server he essentially nullify the L2 economy structure which is in it's core pretty hard (adena, aa, craft) and they replace it with something new and easy.
However this is not a healthy economy structure and inflation hits in a couple of weeks.
L2's expansions where designed to last like 6-12 months and after the content was affected by inflation they just added more content so
they widened the economy cycle more and more.
The easy way to prevent inflation is wiping the content, the hard way is making your own expansions.
If a server doesn't have one of these two elements then it is doomed to fail.
Sure you can make it super hard but then people won't play.
TD;DL: It's not the features of a server that make it last. It is the economy structure and the advertisement investment.