Going by the reactions on seiyuu, the consequences appear to be very harsh to people living in Western cultures. After all, it has not been proven that he knew that the script plagiarised Mitani-san's work (although whether he knew that a script had been plagiarised seems likely, given that he has taken that much responsibility for it). Still, going by the standard Western ideal of justice ("innocent until proven guilty" - which nevertheless doesn't seem to apply in many cases of US politics these days), many fans might find the punishment overwhelming.
However, many Japanese (and other Asian) fans (of Mitani-san) have reacted strongly, claiming that the punishment is TOO LIGHT, that "all work" should have been taken literally, regardless of the trouble it would have given to the companies. In all honesty, I expect that some of the companies might have preferred that as well, as they might not want their anime associated with an artiste involved in such a scandal, or more simply, because they wouldn't want to offend particular important people (like Mitani-san) or groups. The important concept here is the notion of 'face' (which, come to think of it, might have been a major reason for the Earthsea problems I've mentioned previously). And in our cultures, money cannot compensate for slighted honour.
I'm disappointed and saddened by the whole situation. By several reports (I'd credit the person who provided them, but I don't think she wants it broadcast that she reported it), Sakurai-san's behaviour in the past few months or so hasn't exactly been admirable, although he seems to have been under a really heavy workload the past few years, which may have contributed - photos from the Advent Children premiere last year gave me a big shock - he looked very gaunt then, and going by the Code Geass video I saw the other day, he still doesn't look as healthy as he was back when I first noticed him (mid-2004). Still, his alleged arrogance rankled, and I expect many of those 2-channel fans who criticised him for it to be rather pleased with the hole he has fallen into here. For his fans on the other hand, how much culpability he actually has for the situation will probably be important, and this is something we will never truly know. I don't believe he knew it was Mitani-san's script, and I sincerely hope he was not even aware that the script was produced by plagiarism, but the latter seems unlikely. (Googling, of course, would not have helped at all, simply because this script has only been published once, about a decade ago, and it seems like it has been kept pretty tightly under wraps since then.) Either way, as the leader of the theatre troupe, he has had to take responsibility. If he was indeed unaware, then the punishment really is very harsh. However, if he knew that a script was being used without the permission of its writer, then the punishment really is too light. For someone working in the public sphere (even if in a country that has many grey areas wrt copyright), taking someone else's work without credit is wrong.
Tribulations have a purpose, and I hope that Sakurai-san learns from this one. It will be difficult for him to continue working with the criticism of so many fans/anti-fans reverberating around him, and it might be uncomfortable for his fellow seiyuu as well. The fact that several of them have expressed their support for him will hopefully help, and support from his fans may as well. However, as nanya_hime said, it would be better if he really disappeared for a few months as a show of repentence, then come back and try to start again. Even though he will never regain the position he had before this, the public and others in the industry may be a bit more forgiving if the punishment is heavier.
(On the other hand, some issues that have been discussed in my "language and culture"-type classes at uni spring to mind. In Chinese (and presumably Japanese and other Asian) cultures prior to their encounters with Europe, the nature of scholarship was very different (and the influences can still be found in the writing of their people's today). In Chinese culture, it is (apparently) not important to credit other sources, if only because the use of particular phrases and expressions "covers" that already, because most of those phrases and expressions, and even ideas, are/were widely known to have come from particular sources. You could say that the influence of Westerns ideas of intellectual property, largely based on the use of that IP for personal profit on some scale, is a corrupting influence on what was traditionally a culture that, despite the sins of many of the officials in its political system, is communal as opposed to individualistic.)
For a contrast in fan reactions between East and West, check out the story of Cassandra Claire. I only came across it a couple of months back, and haven't looked at it in detail, but it was a huge scandal in the Harry Potter fandom. Admittedly, CC was only a BNF, not a real celebrity in the public eye, but I think the various reactions say a lot.
I was going to do an Ouran write-up by today...but it may have to wait.