This sounds like Monday morning quarterbacking. It's not like it is feasible to increase capacity by a couple thousand every couple of years. Construction projects are hard to organize, hard to fund, hard to implement. They saw an opportunity to increase capacity. They increased it more than we need. But not obviously more than we would need, nor — for that matter — more than we might yet need in the next few years.
The argument that it's better to sell fewer seats at a higher price is fair enough economically. We could sell out a 30,000 seat stadium every game, and who knows how it would change overall revenue. But you also want to increase your fan base, intimidate opposing teams, inspire recruits, etc. Bigger is definitely better for those purposes.