There's good arguments on both sides. We all know nanos going as small as 1bbl can be profitable. Hell, Hess is moving up to a 30bbl system from 1.5bbls, but that's mostly on investor money and loans, not from profits from the brewery.
That said, the anti-nano folks tend to totally ignore the extreme low cost of entry for businesses like this. A determined DIYer can get into this business very cheap, the trick is making your operation not LOOK cheap so that people respect you as a business.
The other thing people fail to note is that the Tasting Room model makes a very small brewery much much MUCH more feasible. If you're a packaging brewery and selling kegs to local bars, you can sell those kegs for ~$150 depending on your market. If you're selling all that beer directly to your customers, you can sell the ~110 pints in that keg for $4-$5 dollars apiece, which means at least $440 per keg. Selling smaller tasters and half pints increases this profit.
Large breweries can't work exclusively on tasting room sales, they have to distribute to get rid of all their beer. Most people aren't going to drive to a tasting room to get beer, so packaging breweries take advantage of a much larger market, but small breweries can sell more of their stock at much higher margins.
One last thing is that large systems (even as small as 3bbl) take a pretty big time and money investment just to LEARN how to use/clean/maintain/troubleshoot and come with a whole host of new equipment and problems. Think glycol systems, steam lines for heat, automation, CIP systems, and the whole range of dangerous chemicals and OSHA requirements that go with them. Same goes for a large brewery needing to have lots of employees- think employment law, workers comp, OSHA again, payroll, tax, and all sorts of other nuisances a sole proprietorship doesn't bother with.
In short: We know the nanobrewery is a workable model because there are already tons of profitable nanos in existence. These guys don't want to deal with them because of the high percentage of tire-kickers (as mentioned earlier, thanks to the nano boom) and small profits to be had. That said, I agree with the skeptics who say that you all are going to be able to pick up 1-3bbl systems for very very cheap in a couple of years. You can't have mine, though.