...I don't think Google has anything to fear from any other search engine...at least not for the next ten or twenty years minimum...they have an edge over the others like MS has with Windows in the operating system area...we know how long this can last now
as to running an extension or a script, many will use that...will be interesting to know the percentage, and it's obviously too early to tell...I doubt this will have the same success like AdB+ has...
To tell you the truth, they wanted to bring a new design allowing to reach anything on the web, and I'd say "mission completed so far". The rest isn't very interesting. People will learn how to use the new interface and it won't take more than a couple of months until they hate the old version, or just forget it ever existed.