Here's the thing. Shorter innings doesn't mean shorter commercials. If we have 9 inning games, and commercials every half inning, would commercials every half inning get shorter? Probably not. Technically, they're only going to need to shorten the content. We would see fewer commercials if there is a limit to bullpen changes, but that might be one string of commercials per game.
As far as stadium revenue, I don't think it would take a big hit either. Right now, your average fan doesn't get into the stands until the second or third inning anyway, and the average fan leaves immediately after Sweet Caroline. Cutting night games from 7-10:30 to 7-10 will mean that more fans get in the park on time, and that families and 9AMers are more likely to stick around for the whole game. It also means that stadium workers will be paid for fewer hours. Worst case scenario, beer goes from $10 a pint to $15 instead of the expected $14 hike.
I think the 18 minutes was actually referring to football. With baseball, its closer to 5-6 minutes . I had proposed a 3 ball rule in another thread, which would be a far more drastic and unrealistic change. More offense, more contact, fewer pitches thrown, less nibbling.