I do feel those wanting the NA release to be the same servers as JP haven't really looked at the logistics and that goes for @N-Cali-Man too...
Whole worldwide shared server would only be possible had the NA version come out when the game was still new in Japan. Right now with all the content exclusive to Japan there's just so many things that would be awkward...
-
PSO2 has several features that really only work well with Japan. So far there's items you can only get through special codes distributed through Japanese-only products, there's a counter which gives you exclusive items if you are playing the game through a SEGA Partnered Internet Cafe (these sort of things are common in Asia, having features limited to this is something no game in the West has done to my knowledge).
-
The people currently playing the game already on the JP server do not represent the entire Western fanbase. PSO2 NA's release is being treated as a "new game" release in the West with changes made to accomodate the fact that to many this would be their first exposure to PSO2 (whether they succeeded with this or not is another story). Imagine the confusion of many new players simply being dropped onto whatever it is JP's doing right now...
-
Carrying on from the last one, we also have all the past content to deal with. Consider anniversary events, JP's heading to its 8th Anniversary event while all the new players on this 'new release' (not new 'game') cry out over missing all those past content!
-
Again, the JP version is and has been managed with the JP audience in mind. There's collabs with games and anime that aren't/haven't even been made available in the West. Not to mention all the licensing issues for many of these...
Lastly... the whole split server arrangement has been something ongoing since Phantasy Star Online Blue Burst through titles such as Phantasy Star Universe and Phantasy Star Portable 2. While it worked less than ideal for those titles, given the current scenarios I can't imagine any other way of launching PSO2 in the West and at least hope that, even if the servers are separate, NA keeps up as best it can with JP and gets its fair share of content from the JP version as well as new content made for it. (PS Portable 2 had Western exclusive content to replace some collabs that couldn't make it over)