Money doesn't matter between the white lines in the last year of a long contract.
And platoon =/= everyday.
Adams would be your option against RHP.
Nationals first baseman had 1040 plate appearances in 2018. 783 were against RHP. So, 75% of the time, your first baseman will face a RHP. So that means, 75% of the time, Matt Adams should be hitting. That's basically an every day first baseman.
Like I said, if anyone thought Adams was worth starting, he wouldn't be a bench bat.
Money definitely matters and so does the perception. Also, when Zimmerman is healthy, he is a MUCH better hitter than Adams.