I could buy the dream theory if it were applied to the series as a whole. Mild-mannered teacher desperate to take care of his family after learning he has cancer falls asleep and dreams his rise and fall as a meth kingpin, populating it mostly with people he knows: a former student, the sleazy lawyer in billboards all over town, the manager of a chicken restaurant where he may have eaten before, etc.
It would be lazy, terrible storytelling, Dallas x 1,000,000. But it would make more sense than, "Everything after the cabin is a vision, because it implausibly works out for Walt in the end." EVERYTHING works out for Walt (as he destroys everything in his wake), from Krazy 8, to the fulminated mercury, to Hank conveniently showing up to kill Tuco, to the nursing home bombing. What about any of those scenarios is not unbelievably convenient for Walt?