One reason to replace what we have is what we have is getting old. I know I was surprised to learn that Highmark (meh) Stadium is older than War Memorial was when it was replaced. And those of us old enough to remember that dump believed it was built by the Romans.
But the real reason for a new stadium is money. It's why the NFL has been pushing for one for what seems like forever. As far as the league is concerned, Bills tickets are too cheap. And you can't simply double prices in your current stadium. You have to build a new palace.
And it's more than just ticket prices. Other than charging you for parking, teams don't make money off tailgating. Instead of acres of people eating food they brought with them, replace it with bars and restaurants in and around your new palace, some of which the team owner might have a stake in.
So get ready for change.