Donald is rightfully trying to break the bank. He is by far the best player on Defense in football.
His fifth year option and the Rams franchising him is still a bargain for the Rams.

1-Why sign him if it doesn't benefit the teams cap as much as the fifth year option and a couple franchise tags (not ridiculous for DTs)?
2-Why Peyton Manning him....meaning kill your cap and quality depth so you can't sign Gurley and Goff.
3-He runs out of Franchise Tags when it's time to tag Goff.
4-After his fifth year option sign Gurley or replace him. He'll be cheaper than Donald, but five years in won't he start his decline? Not worth a long extension.
5-Trade? Not for a couple years.

My point is signing Donald while winning and having depth may be impossible. Keep him until you can't.