You do it because it shows good faith and a deep commitment to a franchise player, who of course knows that he is under-paid and would deeply appreciate the gesture from the team.
You do that because locking him up and demonstrating this type of good faith sends a great message to other players currently on the team, and others who might ever be interested in joining the team.
You can show good faith while still waiting a full 2 years which would still be 2 years before he is due for a new deal.
What will lure others to the team is cash. Sadly that is how it works. Right now Goldy's status as seriously underpaid is an advantage in the pursuit of free agents and the money required to land them. If anything getting locked in to paying a guy 150+ million will hinder, not help their pursuit of other top talent.
If he was hitting FA this winter that contract would be robbery. Keep in mind Pujols and Cabrera got 250m at 32. Goldy is 27. If the Diamondbacks want to make Goldy a lifer they need to pay him like the elite player that he is.
And Pujols' contract was considered one of the worst in all of baseball by the time he was 33. And Cabrera, while his contract extension hasn't even started yet, Detroit has had to hold a sell off, and couldn't hold onto Max Scherzer because they have so much salary locked in.
People were surprised at the timing of Cabrera's extension because he was still locked in for 2 more years. Goldy is signed for another FOUR years. Extending him now makes absolutely zero sense for the Dbacks. I'm not saying "never extend him", just WAIT! There is no harm in waiting another 2 or 3 years and you'd still be doing good by the player by extending him early. Where to do it now would be foolish.
Of the guys who have signed these mega deal, 25+ million per year 5+ year contracts, virtually all of the franchises to land them have regretted it immensely. Typically playing out as several seasons of dead salary rate or having to pay the guy to play for someone else during the later half of the deal.
Right now Goldy is on one of the best contracts in baseball, maybe THE best contract in baseball. If they tacked another 6-7 years and 150-200 million dollars onto the back of it, playing him from 2020-2025 now its instantly one of the riskiest contracts and a virtual guarantee to be a terrible deal during the final 3-4 years of it.