Brian Cashman wanted to trade Robinson Cano in his 2013 walk year. The same with David Robertson the following season.

Not until 2016 did his bosses fully buy what the Yankees GM was selling — that modern rosters must be young, deep, versatile and financially flexible, and that maintaining George Steinbrenner’s model of never taking a step backward would guarantee not one step back, but a free fall.

That revelation arrived at a most fortuitous time. The Yankees had Aroldis Chapman and Andrew Miller to trade when every contender wanted bullpen pieces that looked just like Chapman and Miller. That allowed the Yanks to set high opening prices before a subsequent bidding war that netted Gleyber Torres, Clint Frazier, Justus Sheffield and more.

The Mets face a similar situation of both free fall if they don’t change their ways and perhaps an antidote in Jacob deGrom and Noah Syndergaard. Except this July market is going to hunger for deGrom and/or Syndergaard even more than the 2016 one did for Chapman and Miller. Texas’ Cole Hamels, Toronto’s J.A. Happ and San Diego’s Tyson Ross project as the likely best starters available, and the difference between them and deGrom and Syndergaard is Kevin Durant to Kyle Korver.

Mets officials have indicated to me they would be crazy not to be open for business if their season continues to plummet, though I sense if they took a dramatic step they would only want to trade deGrom or Syndergaard (probably being more open-minded to Syndergaard) and retain the other because they want to avoid a full rebuild. They also signaled they would consider the Yankees, though the two New York teams got contentious about near deals last year for Jay Bruce and Neil Walker. DeGrom/Syndergaard would be a different weight class and more fraught with tension.

And here is what Mets fans should remember: Making someone available does not mean having to trade him. The Mets also can set a high opening bid and see where it goes.

Consider that for three months of a closer, Chapman, the Cubs surrendered a four-player package headlined by Torres. So if, for example, the Yankees want three pennant races out of a low-maintenance/high-performing ace such as deGrom (free after 2020), the starting piece should probably have to be Torres.

All of you Yankees fans about to make suggestions like Frazier, Tyler Austin and Sheffield for deGrom, understand that you wouldn’t do that if you were the Mets GM. To get someone like deGrom or Syndergaard (not free until after 2021) would entail great pain, which means starting with Torres or Miguel Andujar or maybe both. It is a pain I cannot imagine the Yankees enduring.

But the Yanks face pressure, too. They have a near future of championship contention with one worry spot, rotation quality and depth, exacerbated with Jordan Montgomery needing Tommy John surgery. Despite a large talent gap between the New York teams, the Mets actually have a starting advantage in the first two Subway Series games this weekend with deGrom and Steven Matz vs. Masahiro Tanaka and Domingo German, and don’t have a bad counterpunch to the Yankees ace, Luis Severino, with Syndergaard in the finale.

Imagine, though, if the Yankees could open a playoff series with Severino and deGrom — their title chances in the next three seasons would rise emphatically. That is why the Mets’ request should be so high to the Yankees or rotation-hungry contenders such as the Braves, Phillies, Brewers and Dodgers, who have deep systems and rotation needs. DeGrom or Syndergaard could change championship fates as much as Chapman and Justin Verlander did the past two years.

And the Mets must think about it, must look beyond their propensity to prioritize selling tickets come August or next April.

Sandy Alderson has expressed that not every full rebuild works and that they are long and painful even when they do. But you know what also won’t work? Just trying to fix at the margins around a talented but fragile rotation. Not when the current positional lot is older and unathletic and the farm system is in far worse condition than the Yankees’ was before the Chapman/Miller deals.

The Mets can wait a few more weeks and see if they reverse into a contender, think further if it is best to build around deGrom/Syndergaard rather than break it up.

But they are going to have no choice but to be open-minded and open for business. The market is craving arms, and the Mets have a strong late-inning piece in Jeurys Familia, the kind of multi-use weapon teams want in Seth Lugo, potential rotation depth items such as Matz and Zack Wheeler and — most of all — two ace-level talents in a marketplace that looks as if it will lack just that. This is called opportunity. It knocked for the Yankees in 2016, it is doing so now for the Mets in 2018.

Will they answer?