Part of that, just like football, is that no one gets a chance anymore to build a program. It's 3 years or you're gone. To me it's a major flaw in college sports today .
I agree, but to be honest if a coach hasn't seen big improvement by the end of year 3, it probably isn't going to happen.
If you take a look at past and present top coaches - I pulled up Coach K, Bill Self, Roy Williams, Calipari, Jim Calhoun, John Thompson, Boeheim - they had their programs "up and running" by year 3 with the exception of Coach K. That's not necessarily winning titles by year 3, but they were competitive by that time.
It actually took Coach K 4 years at Duke to even make the tourney, and you are right, if that happened today a coach in a similar situation to the one he was in would be run off after 3, probably even 2 years of that.
If the climate today was like the climate in Bear Bryant's time, Bryant would have been run off at some point during the 1967-1970. It is easy to look back on legendary coaches today and assume they fielded title-contending teams every single year, but they simply did not. Many, if not all of them experienced lulls in the program that probably would get them fired today.