From past experiences with new players, I’d say EVE typically has about 1-2 years worth of ‘cool new’ stuff before they start running out of things they’re willing to spend time on.
Some will quit well before and some will stay long after, but that’s about how long it takes on average (my anecdotal experience, not data backed).
For themepark MMOs, the problem becomes “how fast can devs add content vs. how fast players burn through it”. Typically 3-4 months of development provides about 2-4 weeks of “new content play”. So the devs are always running behind, and “the grind” is intended to be there to fill in the gaps between releases.
With EVE as a sandbox, the players are supposed to be able to provide content for each other and use the tools within EVE to set up situations that make them want to keep playing. The problem is that the big ones (conquer Null or WH areas and set up your own empire) are pretty much already done with.
And the smaller ones (Incursions, Faction Warfare, Resource Wars, wardecs, exploration, missions) are either broken and left that way for years, or over a decade old and haven’t been touched.
If EVE had a wider appeal, and a decent monetization model (ie., more things they could sell - EVE is very limited in monetization options), then they could survive on the high-turnover, churn new players, make $25 average off each one model.
But it doesn’t have that appeal or that model (although that’s where CCP is trying very hard to go) and so it has relied heavily on the past on the core dedicated players with long-term subs, multiple subbed alts, and willing to make Plex/skin purchases.
What CCP should be doing is putting more sandbox tools in the game (fix FW, Resource Wars, liven up Trig minor systems, make Low-sec more interactive, add more Corp tools and Contracting options etc.) so that players could do exactly what EVE was supposed to do all along - give players the chance to create their own stories and interactions.