PHOENIX — Emmitt Smith is so tired of the devaluation of running backs that he will cross Giants-Cowboys rivalry lines to praise Saquon Barkley.

The NFL isn’t played the same as when Smith retired as the all-time leading rusher in 2004, but nothing has changed more than the school of thought that top running backs are not worthy of big contracts because they can be replaced with cheap, young rotations. Barkley is the co-headliner in a deep free-agent class featuring reigning rushing champion Josh Jacobs, Super Bowl LVII starter Miles Sanders, Kareem Hunt, Tony Pollard and David Montgomery.

“It’s the narrative that the NFL wants, so they put it in the head of everyone else on talk radio and promote it that way,” Smith told The Post. “My problem is that narrative is not factual. When you have built the game to protect scoring points and you are going to protect the quarterbacks and the wide receivers, but you are saying the running backs cannot do that. You have yet to give the running backs a chance to be bell cows like they want.”

The conversation shifted when Todd Gurley’s record-setting extension with the Rams in 2018 turned out to be a disaster, and the Chargers forced Melvin Gordon to play on the franchise tag in 2019. The early returns on Ezekiel Elliott’s and Christian McCaffrey’s extensions were not good, either. Analytics show running backs have a shorter shelf life because of the wear and tear on the body.

But Derrick Henry and Nick Chubb — who finished second and third in rushing yards this season, right ahead of Barkley — plus Dalvin Cook, a resurgent McCaffrey, Alvin Kamara and Aaron Jones all are dynamic playmakers making $12 million to $16 million per year on extensions.

Barkley, who finished third in Comeback Player of the Year voting after two injury-plagued seasons that included a torn ACL, already turned down a multi-year contract averaging about $12.5 million per year but negotiations are now simultaneous with free-agent quarterback Daniel Jones. Smith held out into the 1993 season and became the highest-paid running back when the Cowboys started 0-2.

“I love Saquon,” Smith said. “He had a great bounce-back year and no one is patting him on the back for the mental toughness and the fortitude to work offseason-wise to get your knee back and perform the way you did. They expect that.

“Yet they don’t expect the Giants to give him the ball as much or they don’t expect the Giants to do the right thing to secure him. They want to ship him off. You have to strike when the iron is hot because they are using you for your talent, and they will discard you when they don’t want to use you anymore.”

Why was relatively the underpaid backup Pollard able to out-produce the highly-paid Elliott for the Cowboys this season?

“The talk is about money, but talk about performance,” Smith said. “Tony makes plays happen because he sees soft spots, but when Zeke is in the game people are expecting him to get the ball. Psychologically, it’s bull—-.”

The subject is especially close to Smith’s heart because his son, E.J., is a versatile running back at Stanford with NFL aspirations. They appeared together on Radio Row as “wingmen” for the promotion that the Super Bowl, wings and Bounty go together.

“Saquon does more for the Giants than people realize,” Smith said. “Just his presence on the field makes the defense do certain things. First-, second- and third-down backs exist today in a much more broader form, but when you have a narrative that backs are not valuable it gets regurgitated out of the mouth of babes. Nobody is willing to change the conversation. Everybody wants to hide behind numbers as if they are absolute truth without telling you what’s behind the numbers.”