I believe Root gambled on his bowlers, particularly Anderson and Broad, based on the expected weather conditions. I think that's a fair decision given England rely on Cook and Root at bat, Anderson and Broad with the ball, and Moeen and Stokes in-between. It's looking like the wrong decision but I can see why he made it, especially given Australia have won day-nighters at Adelaide fielding first. Nevertheless, it's proving wrong and just about everybody and their dog would have chosen to bat first.
Regarding the follow-on, it's really fallen out of fashion over the last couple of decades. I think a lot of that is to do with the quicks being asked to push harder, so they physically need the rest. Plus pitches generally, and this one in particular, are going to benefit the bowlers come the fourth innings. A lot of talk relates to the 2001 Test between India and Australia, where Australia enforced the follow-on only to face spectacular batting by Laxman and Dravid, and then bowling by Harbajan and subsequently lost. I think that's misleading and it is essentially about resting your quicks and taking advantage of the deteriorating pitch on fourth innings.
All that's just my opinion. The beauty of cricket is that there will be perfectly reasonable and fair points of view that are in complete disagreement to mine