I suspect part of the argument against new space-sims is the ongoing failure of the genre - Earth: Above and Beyond is the game which replaced Privateer Online so far as 'online space sim' goes, and it's being shut down in September due to lack of players. Jumpgate and a few other such games are still around, but they don't draw the massive numbers that UO and EQ do - heck, even Planetside seems pretty popular these days.
Granted, half those games are either poor designs, or were not fully realized (IIRC, EVE: Second Genesis has still been adding features since it went live last year), but it still is a bit disheartening, at least when you realize the companies who WOULD publish such games look at those results and decide the return on investment just isn't there.
It's one reason CRPGs were ignored for so many years, at least after the Gold Box series of games ended and games like Baldur's Gate and Fallout 2 helped revitalize this market on the PC, as publishers realized that these games COULD sell well. The space-sim genre hasn't had this in about eight years now - the last game which sold really well, IIRC, was WC3. Things have slid since then, even with the X-Wing games from Lucasarts: X-Wing Versus TIE-Fighter was probably their best title ever, as far as their space-sims go.