The chances of Iowa winning a national championship in football are astronomically small. BUT, should that ever occur, then yes, Ferentz naysayers should never speak ill of him again.
HOWEVER, germane to the real issue is the overall results and health of the program. It's easy to make misjudgments about a program based on wins/losses alone. The real question, in my view, is: where is the program going and does it pass the eye test?
The Iowa program will likely never return to having a 3-year run like we saw from 2002-2004. And, in fairness, that's a pretty inflated expectation about anywhere in the country outside of maybe Ohio State and Alabama. Regardless, probably the best view of the Iowa program is from 2005 to the present (with 1999-2001 and 2002-2004 obviously being the anomalies or outliers). So, if we judge the program from 2005 on as the most accurate measure of Iowa football, which the pattern has been pretty consistent since that time, then we see Iowa is 100-66 in 13 seasons (with this year's bowl game yet to be played).
If you do the math, Iowa is averaging 7.7 wins a season since 2005. Based on the wins per season alone, that's not great, but not too bad, either. However, what the average wins per season don't factor in are one) the schedule strength every year and two) whom those wins and losses occurred to. Clearly, Iowa has won some big games they shouldn't have during that time frame, and in turn lost quite a few games they probably shouldn't have.
To me, that is Iowa football under Kirk Ferentz. You get extremely wild, inconsistent results--more than probably what the norm is for the rest of the country. I think that is what is terribly frustrating for a lot of people. And, depending on the type of fan you are, you either focus on the positive end of the spectrum (Orange Bowl win, upset wins over Top 5 teams in Kinnick, and a 12-0 undefeated regular season) or you focus on the other end (losses to MAC teams and 2-10 Iowa State teams, below average recruiting, and terrible offensive numbers).
As I've said many times, I personally have been ready for a change since 2012, namely because I think the product on the field is stale and boring. However, since Iowa is locked into a contract with Ferentz for the foreseeable future, fans have to accept Iowa football will be wildly inconsistent in terms of the results. Next year, it is certainly possible Iowa can have one of those improbable 12-2 type seasons to be followed up with a 7-6 campaign in 2019. That's how it works and it likely isn't going to change.