He's not assured of 40 now. Without a contract, he's only assured of 20.
Your assumptions about not making it up disregard the potential for injury. Personally, I'd rather take a guaranteed 40 or more for 2020 & 2021, than 20 for 2019 with no guarantee for anything beyond.
And yes, I'm talking about sitting the entire year What the Bell situation taught us is that accruing the year is meaningless under the 2nd franchise tag. Playing those 6 games is even more risky because now you're risking 40 million for just 7 or 8 million in salary for those games.
You don't accrue the year, but the 2nd franchise tag still counts.