Here's my standard Machado response.
Put yourself in his shoes.
You are 25 years old and have the chance to enter a free agency that makes you one of the most pursued players in a free-agency packed with big salaries and fellow stars. This free-agency period has been hyped for years in the making.
So, with baseball lining up to make you swoon, why on Earth would you agree to an extension with the team that trades for you?
That's the most realistic way the Cardinals could get Machado, and it doesn't seem that realistic.
If the Cardinals don't have a good feel that they could re-sign Machado, it's not worth trading whatever it would take to get for him. Not for half of a season.
And I'm sorry, but I don't see any recent history that suggests the Cardinals will win a bidding war if (when) Machado hits the free-agent market.