Québec still has the third highest GDP in the Canadian provinces [1]. Québec lost a lot of manufacturing and automotive sector jobs in the last decade or two. So has most of the western world. I don't think it has much to do with the language.
As for the infrastructure, I'd posit that crappy infrastructure at high costs is largely due to corrupt politicians (ever hear of another place where money played too big a role in electoral campaigns?) and construction companies being run by the mafia [2].
Which, of course, all are problems of their own, and in no way is Québec a utopia. It's just a bit tiring to always hear the easy intellectual shortcuts that blame everything on law 101.
[1] https://en.wikipedia.org/wiki/List_of_Canadian_provinces_and... [2] http://www.theglobeandmail.com/globe-debate/editorials/the-c...