When your current highest paid player is a 33 year old 3B with an extremely checkered injury history? Doesn't seem to look quite so bad then.
Would it be a gamble? Sure. The difference is that Greinke has consistently proven that he can be a front-line SP with his diminished velocity - we're not talking about Julio here. He still misses a lot of bats, still doesn't walk anybody, and still doesn't get squared up very often.
I wouldn't blow the farm for him either, but if they ate the deferred money he wouldn't need to be THAT good to earn what you'd be paying him. The bigger question is which one of the current staff menbers/prospects do you have more confidence in being a 200 IP, 3+ WAR Pitcher for the next 3 years? Like everybody else, I hope Folty, Gohara, Soroka, and Anderson could - but we don't have any guarantees they will.
To me, it's more about the realistic window being open from now through 2021 - after that you have to make a decision about how much production you hope to get from a potentially declining Freeman.