Electing not to defer just so you have a 1/1000 chance of the other team making a ridiculous mistake is silly. There are analytics that give you tangible reasons for receiving or deferring.
And the numbers I listed don't lie. Teams are NOT getting an extra possession. Kelly is. That's just a fact. The "being amped up" and making mistakes on offense doesn't hold water either ( again, backed up by the data). Kelly's teams are scoring at a higher rate in first possessions compared to all other possessions.
Also, not every team should follow the same strategy. It's not one size fits all. Each team has strengths, weaknesses that could dictate a different approach.
It's fair to say we don't know what the results would have been if he always deferred. But, we can compare productivity on 1st possessions vs overall productivity. We can also see the result of gaining extra possessions (which the data shows) when taking the ball first.