Correct, the $71K is locked in for all four years. The only variable that could change are grants funds, as determined by annual fluctuations in your FAFSA and/or CSS profile.
If you don’t select the guaranteed option, then you are subject to whatever year over year tuition increases may occur as set by the board of regents each year.
I did the math both ways, one guaranteed and one using the estimated tuition increases over the next 4 years. The guaranteed tuition option for us is less expensive over 4 years. However, the yearly cost is initially worse for us given our tax situation and other factors.
The guaranteed tuition option is so much that we are actually seriously considering her other top choice school. It puts them pretty close together financially.