I came here because i wanted to know peoples opinions on the matter. You are all sticking to the fact that only 10,000 copies were made and all were being used. While this is true I think that we shouldn't go on facts for this. I believe we should theorize and just get away from that all copies were in use. Lets say that about a hundred or so never logged in on the first day. Would they be able to log in if they still had the copy of the game? While we are at it lets take away the whole thing about how the government would confiscate those copies as well. Also lets put peoples reasoning about joining a death game aside. So my answer to this is kinda going to be everywhere.
Alright I believe that in a way they could. I mean look at how the game was set up. Lets start with the Beta of SAO. Everything is great a few bugs here and there but everything is running smoothly. Now for the Full Version. The game was specifically modified to not have a "Log Out" button. So the people that joined SAO day 1 already logged in while the game had no "Log Out" button in the first place. So there goes that theory about people not being able to join after the "Log Out" button was removed. Right now its looking as if they might be able to. Now lets go into the games system Cardinal. The Cardinal system is SAO's whole system and does a number of things. It would also adapt to situations. Lets say one of those things is to make the game fair for everyone. Would it not block out any other people from entering because everyone had a head start? It would possibly try to send them to another server for when the game had multiple of them. But then what about the Beta Players they are ahead of everyone, would that not be the same as if someone new joined? So there goes that one. It would go without saying that nothing is standing in the way of a person from joining. The servers wouldn't be full so people could still join. Unless the guys that worked on the game made it so that no one could join. But with that said if they had that power to do so than wouldn't they be able to pull everyone out of SAO? It's most likely that Kiyaba was the last person to check the systems before release, so he could have made changes to the game that the others didn't notice. Or maybe Kiyaba was the last person the game went through before release so everything would be set up the way he wanted it to. That would also go without saying that only Kiyaba could access the game as we learnt that he was actually playing himself. Now there's another thing we have to consider. I believe Kiyaba would not have joined SAO day 1 either other than his appearance at the opening ceremony. I believe that he would have joined after the first month or so to ensure that everything was going well without him interfering. Also whats to say that Kiyaba thought about people not joining day 1 and didn't bother to put a restriction on it. He did want everyone to join so that he could control a world by his design. So reasoning stands that yes they could join and be trapped there like the others.
That's my theory but something else bothered me as well. For this I am bringing back the rule about the government confiscating the copies. If these copies of SAO were use-able would the government not try to find a way to get everyone out by looking at the games codes and crap. This was most likely a thought in Kiyaba's mind. So he probably did put a restriction of new players joining. Now with that in mind Day 1 would have been the only time to join unless your the GM. Joining on any other day would result in a connection problem. So If we add in that one little detail than the whole answer turns around. So it varies on different occasions but my final answer is no.
Now don't go hating on my theory as this was actually my first theory that i have thought of myself. And I'm sorry in advance if any of it does not make sense as it is very late so I'm tired and I've spent like over an hour writing this.