Optimism aside, because that's what all fans have pre-season, what do you base your conclusion on? BD has no track record as a head coach. He talks a good game but there is nothing to suggest either instant success or instant failure. No one knows how he will call game. Does he turtle once there is a turnover? Does he play ball control to shorten the game for the other team? Does he play to keep it close and hopefully eke out a win at the end? Is the offensive system playing to the strengths of the available personnel? Does he make in-game coaching adjustments?
No history = no expectations. By week 4, Uconn fans will have a better sense of what they have in terms of a coach and his philosophy.
The guy isn't an 18 year old with no track record. He played at a high level in college (he wasn't a 2 time all B10 selection at linebacker 'cause of his 4.55 40 time), was a top assistant at a top program and attracted assistant coaches who have track records of performing at P5 conferences to a non P5 school - they are betting their next career step on him and they should have pretty good insight into him as a potential head coach since they all coached with him. He got a top S&C coach to come from an SEC school.
The previous PP/GDL coaching/preparing/depth chart set up and playing time was so bad that just by subtraction of negatives the team is much, much better.
He makes good decisions. He saw the sleeping giant at UConn while the assistant at MSU just saw issues. He got his coaching team together and has hit the ground running. Instant success is playing solid fundamental football with few mistakes that improve with each game - wins will come in year 1. Last year was 3-9; individual ability level probably same this year but skill/endurance/toughness/performance that will be shown will be much better.
Last year's ending starting QB was 6'0" tall and 247 lbs and now is 217; I am 6'2" and was 251 Jan of 2013 and am know 212 - I can tell you what a difference that makes and no crazed 300 lbs guys are chasing me. And CC wasn't the only one, McCummings was way overweight also.
Diaco will be successful right away. There is nothing to suggest instant failure, a lot to suggest instant success.