If Stafford continues to ball out for 4-5 years, which is unlikely, the Rams have the option to do what the Pats did with Brady and Jimmy G. Towards the end of the contract, you trade the #2 for a 1st or 2nd and get much of your investment back.
Or if Stafford is finally retiring at that 4-5 year mark, since Simpson would be light on experience, you sign him to an extension that's 40% of what a starting QB goes for ala Malik Willis.
In the meantime with either scenario, you have a top-rated backup that keeps you on track to contend for Super Bowls if the #1 misses any time.
The ultimate goal with drafting him now is to have a prolonged SB-contending window, no matter what happens with Stafford.