The reason I mentioned a "double standard" to you was specifically because of the Di Matteo confusion. Since we now agree that Chelsea actually gave him a permanent contract, we were essentially speaking different languages. If you had used Avram Grant (07-08) or Hiddink (08-09) as examples—successful interims who were moved on—there wouldn't have been any confusion.
However, even in those cases, the clubs only moved on from a successful interim because they landed a prestige heavyweight like Scolari or Ancelotti. They didn't replace them with someone who was "promising" or had "potential" from a lower-tier environment. This aligns with my two main points:
1. The "Guaranteed Upgrade" Rule
If United can bring in someone who is obviously better, like Enrique, I’m all for it. But Iraola has no obvious edge over Carrick; he represents the same level of risk, if not more. History shows that "experience" at a lower level rarely translates to an immediate step-up to a giant. For every rare success like Kompany at Bayern, there are multiple failures like Potter at Chelsea, Moyes at United, and more recently, Amorim and Rosenior. Even Xabi Alonso at Leverkusen, who had a much more dominant body of work than Iraola, failed to step up immediately. You could argue that was due to the notoriously difficult dressing room and the failure of the club's management, but United is also famous for having those exact same setups.
2. The Man-Management Hurdle
If we were still in the slump we experienced under Amorim, bringing in Iraola would be a logical experiment. But we aren't in a slump. We are on an amazing run where the players and the fans respect the man in charge.
Replacing a successful, respected interim with an unproven alternative like Iraola doesn't just gamble on tactics—it gambles on the entire dressing room. This man-management factor is constantly overlooked here. Carrick has already cleared one of the hardest hurdles: winning the players over. Bringing in a new guy with "more experience" at a lower level doesn't mean he will have the authority in this dressing room. In fact, by moving on from a successful interim for a unproven one (even if he's promising), you are setting an extremely high standard and putting a tremendous amount of pressure on the successor from day one.
I’m not saying Carrick is a guaranteed success, and I’m not saying Iraola is a guaranteed flop. The bottom line is, when you have points, goals, and a happy dressing room, you only change the head coach for a guaranteed upgrade. Iraola isn't that; he's just a different kind of gamble.