He's probably ready to start his decline at the beginning of the contract, so 90% is what you can expect for the first 2 years. By year 3, he'll likely be a below average 1st baseman you are paying 30 million/year to.
Why does everyone think he's going to decline because he turned 30? Turner has been extremely productive after turning 30. Adrian Beltre had his most productive years after 30.
Its 2019 and we have modern nutrition and workouts.
I mean, I sort of get it because a lot of infielders do decline around that time, but it's far from automatic. For every Bill Mueller or Ryan Zimmerman, there's an Edwin Encarnacion, Aramis Ramirez, or John Olerud.
And it's not like the Nats have a wealth of corner infielders in the pipeline waiting to tear up the majors, so the alternative would be to find a replacement in free agency. Players as good as Rendon don't exactly grow on trees, and when you can find them, it's rare that they're under 29, so the potential for a decline around age 30 would still exist.