1. c: Some frameworks are good for some things and some for others. Surely this "Thin Server Architecture" is just one choice among others.
a: No actually, it is not a choice among others. It is the total removal of any web framework on the server whatsoever. That's what it is. It is "No View on the Server", and it is "Put the client on the client". If you consider "lowering your development and maintenance costs dramatically" a choice among others, be my guest. But I would generally consider that a question of whether to have a third ear at all compared to the question of where to put it.
a: That is absolutely true. If you have to create an application that has that kind of restrictions, you can't use Thin Server Architecture. Luckily the user base of Gmail and other applications of its kind shows that with some luck there might be a sufficient user base out there for consumer facing applications (at the very least) of that kind after all...
3. c: The page will load in weird ways which users are not used to. Nobody will want to use a web application like that.
4. c: There is no need to go to such extremes. We already have AJAX support in JSF and it works well, for minor things where that kind of 'effects' are needed.
a: Since the idea of using Ajax on a separate client (on the client) is opposite to generating the client on the server for every action, this Frankensteinian technique works well for nobody. It is a little bit like tying a jet fighter to a sinking Titanic. You will get a little lift, for a while, and then all will be engulfed by the next wave.
5. c: If you have to shuffle all data to the client using Ajax, it'll be too much data and the page will be slower that if it is created by the server-side web framework at once.
a: Eh, hello? There is still the same amount of data, no matter how you get it over. Using Thin Server Architecture you get the page in place super-fast, then load and pre-load resources under the hood so that the application acts snappy when the user starts to interact with it. Oh, and you only ship the data after that.
Måns Jonasson's research in this area shows a 33% decrease of server resources when starting to move View logic onto the client, from the server.
6. c: We have already invested so much money and training in our server-side web framework of choice,
a: Well, good luck then :)
Also, there is supposedly companies who provide really great IDE support, of various flavours.
What's more, there already truly amazing solutions for bringing this on big time .
So my take on this is that this is the near future. If you don't think so, and have experience to disprove me, please comment here or mail me, because I'd be more than happy to hear what you have to say.