(If you can't plagiarize yourself, then who can you steal from?)
The stability in Content that EVE has enjoyed is based on horizontal expansion. We basically enjoy endgame content at all times and new content rarely goes obsolete. Contrast that with a game that does Vertical Expansion (WoW being the juggernaut here), where every expansion you get a kick in the teeth and a "gratz, the last 2 years were a waste, now level up again and re-earn all your gear. Go on ... get moving."This is the stability that's allowed me to leave the game and come back a few times.
There is certainly inflation in EVE's economy. Rare items, loot-only officer mods, those sorts of things cost a gazzillion isk. But consider this: The Phoenix that I bought in 2007 is worth about what I paid for it. The hanger full of capship bpos that I used to run through my POS for selling bpcs are worth about what I paid for them. I login after a lengthy absence, and the fit on my Golem still works (it might need tweaked, but it still works).
In no other game I can think of do players change contexts between expansion content over and over and over again so seamlessly.
EVE allows you to go run a lvl4 (2003 content) and then catch an Incursion fleet (2010 content), go sniff out a wormhole to explore (2009 content), or tend to your research agent (2003) or PI empire (2010), clone jump (2007?) to null and hop in a Carrier (2006 ship addition) to participate in a hot drop.
Vertical expansion games usually have a "new area" (new continent, new story act, whatever) where all the action takes place with new dungeons and new tokens/currency/whatever to farm. The old content is abandoned in place; the accolades you earned become meaningless the moment the patcher finishes installing the new expac.
EVE has needed very little vertical expansion over the years. What verticality there has been (to make up a word), has been vertical headroom to a particular system. Capships were vertical expansions to null/sov. I'd count Wormholes as vertical (maybe tangential) expansion to the exploration system. Incursions gave some headroom to PVE, but didn't fundamentally change how people saw EVE's overall endgame.
And I think this situation is a good thing. EVE is the sum collection of many moving parts. Improving the individual parts (even if it includes a little bit of vertical expansion in a localized area) benefits the whole, as long as it doesn't upset the balance of the system.
I also think that pushing the boundaries of the entire sandbox incrementally is a far better path to take than announcing (for example) the release of Tech4 ships that will replace our entire hangars. Some other game studios would be tempted to take this approach; in fact, many see no other path to take. (If Blizzard had done EVE, we'd be on tech52 by now).
I'm glad EVE is the way it is. EVE needs to maintain its roots. EVE needs the stability.