Vincent D'Onofrio's Kingpin is one of the best live-action comic book villain performances of all time, so it's totally understandable that fans would want to see Wilson Fisk return in the Marvel Cinematic Universe, but, unfortunately, Marvel would ruin Daredevil season 1 if he ever returned to the MCU as a villain.
After season 1 of Daredevil, D'Onofrio's Fisk was already a breakout both in writing and in performance, but by the close of season 3, the character didn't only stand as one of the best arcs of all time, but also served as an integral element in Matt Murdock's development as The Man Without Fear.
All three seasons of the now-canceled Daredevil were, in one way or another, about Matt Murdock's struggle with being a vigilante. Or, more specifically, a struggle with the violence inherent in vigilantism. His mentor, Stick, had tried to raise him as someone willing to take the lives of the wicked, but Matt didn't want to be an executioner. He saw violence as necessary, but still felt his Catholic guild tear at his soul, spending much of the first season pondering his own morality through the screen of the confessional booth with Father Lantom.
In Daredevil season 2, Frank Castle's Punisher is introduced to push this idea even further. In the famous rooftop scene, they distill Daredevil's stance on killing by contrasting against Frank's complete lack of concern over taking life. Frank pushes back against the notion that what he does is wrong, yelling: "What do you do? You act like it's a playground. You beat up the bullies with your fists, you throw them in jail, everybody calls you a hero, right? And then a month, a week, a day later, they're back on the streets doing the same goddamn thing."
Matt fires back: "I live in the real world and I've seen it. Redemption, Frank. And it's possible. The people you murdered deserve another chance... to try again, Frank. To try. And if you don't get that, then there's something broken in you that you can't fix, and you really are a nutjob."
While Matt was so sure in his morality at this point, the loss of Elektra and the Midland Circle building collapse in The Defenders left him emotionally and physically broken. Then when Wilson Fisk returns, Matt finds the words of Frank Castle ring true. Simply putting these criminals in prison so they can escape again isn't saving anyone - especially Foggy and Karen, who have come into Fisk's crosshairs.
The whole point of Daredevil season 3 was to push Matt Murdock to the brink. He was convinced the only way to truly stop Kingpin once and for all was to kill him. It's only through the faith (and sacrifice) of Father Lantom, Sister Maggie, Karen, and Foggy that he regains his own faith in justice, having Fisk sent to prison, threatening to get Fisk's wife, Vanessa, locked away as well if he ever hurts anyone ever again. The perfect ending to Matt's 3 season-long struggle with vigilantism, violence, and killing.
Because of D'Onofrio's brilliant performance, and the fact that he's still alive in prison, fans naturally want him to come back again, maybe even in an MCU movie, especially now that most of the Netflix shows have been canceled. While it would certainly be great for fans to see such a great villain return, especially if it's in a movie such as a Spider-Man sequel, Kingpin can't ever come back as a villain, or it entirely invalidates Daredevil season 3, and thus, the entirety of Matt Murdock's character arc.
If Kingpin ever gets out of prison (again), or even if he remains in prison and coordinates any sort of criminal conspiracy, it proves that Matt Murdock was wrong in the end. Going back to the rooftop debate in season 2, it proves that Punisher's methods, the methods Matt almost inherits in season 3, are the only way to truly protect the city.
Of course, that's not to say he'll never appear. The Netflix Marvel universe is likely going to disappear as each show is canceled (pending The Punisher), and they've managed to stay fairly insulated from the mainline MCU, so they could just handwave all that away. Going against Daredevil's character arc could be worth the price to get a villain like D'Onofrio's Fisk onto the big screen, but it will forever taint the climax of Daredevil season 3, and fans will know when Matt screams in Fisks face "you will go back to prison, and you will live the rest of your miserable life in a cage, knowing... I beat you," that he was wrong.