So Data never died then, and he eventually retired from Starfleet and lectured at Cambridge.
Well, no. Data died with the rest of the Enterprise-D crew when Veridian III was destroyed.
Here's the thing about the Nexus: Guinan specifically states that once you're in it, you'll NEVER want to leave. It's so full of awesomeness and joy that you get completely wrapped up in the fantasy, and even if you're still under the impression that it's all fake, it's so overpowering that if somehow you're forcibly taken out of it, you'll do almost anything to get back. And you'll certainly NEVER care again about what's going on back in the real world.
So what do we see instead? Both Picard and Kirk starting their respective fantasies, but in a matter of a few minutes determine that it's all bullshit and decide to leave because what's going on in the real world is more important than this fantasy world. WTF?
Captain's prerogative.