Let's ignore Pep for now (and the possibility that you're on a wum because it could be a truely interesting debate), if we discuss the Premier League today and we should do the same with Klopp, because clearly he couldn't have had a big impact on the league yet.
I've touched on the manager problem in the Premier League before in a different thread and I actually think it's huge. If you look at the age of the managers at what should be the big four English clubs, you have United with van Gaal (64 years old), Chelsea with Hiddink (he's 69), City with Pellegrini (he's 62) and Arsenal with Wenger (he's 66). Compare that to the top performing teams in Europe right now. Barca with Enrique (he's 45), Atletico with Simeone (he's 45), Real with Zidane (he's 43), PSG with Blanc (he's 50), Bayern with Guardiola (again 45), Dortmund with Tuchel (he's 42), Juve with Allegri (he's 48).
Do you see a difference there? If we go back over the decades and look at what age managers usually did their best work, at what age managers created the truely great and era defining teams:
The Dutch managers and their big influence:
Rinus Michels was 37 when he took over at Ajax, won the European Cup at 43.
Johan Cruyff was 38 when he took over at Ajax, 41 when he took over at Barca. At age 45 he had won 2 Cup winner's Cup and one European Cup.
Van Gaal was 39 when he took over at Ajax, won the UEFA Cup when he was 40, then Champions League when he was 43
The Italian managers:
Trapattoni built his great Juve side in his late 30's/early 40's.
Sacchi built his great Milan side in his early 40's
Capello took over and did his best work at Milan in his first job in his late 40's.
Ancelotti followed soon, also in his early 40's.
Lippi did his best work at Juve in his late 40's.
We can go to different nations as well, Germany for example:
Udo Lattek was 35 when he took over at Bayern and built that 70's team that won 3 European Cups
Hitzfeld was 42 when he took over at Dortmund and build the team that dominated the Bundesliga and won the Champions League over the next 5 years
All these names have in common that they did their best work in their 30's and 40's, some were still good or great in their 50's, but in general they never reached their previous peak again.
Many people on the Caf love to belittle Barca for giving young managers a chance and talk like everyone could manage them, but that's shortsighted and false. What Barca is trying to do, what Milan and Juve did for decades during their successful era, what brought success to Dutch and German clubs is something entirely different. They tried to stay ahead of the game, constantly bring in forward thinking young managers with new ideas. The Premier League over the last years has done the exact opposite, just reacted, always one step behind but celebrating the big name managers who might already be past their peak.
Don't let Ferguson's legacy fool you, he's a one off. When the Premier League rose to the top in the 00's, they had forward thinking young managers in Wenger, Mourinho pushing Ferguson to adapt and he did. No one is pushing Wenger right now, no one is adapting. We'll see how Klopp and Guardiola can influence the league. But it wouldn't be surprising if they had already done their best work in the past. I don't think it's the case with them, they look driven and willing to learn and adapt to me, but we'll have to wait if that's really the case.
Sadly, Bayern has often done the same in the past and I've been fairly critical about that. We had some success with it, Hitzfeld was still young-ish at 50 when he took over in 1998. Heynckes was a rather unique case. But Trapattoni in the mid 90's was already past his peak and I've been critical about the appointment of Ancelotti as well, because we missed the chance to bring in a new forward thinking young mind. I hate it that Tuchel is at Dortmund and that the timing was off, so that we didn't get him.
To come back to your original post:
Undoubtly the Premier League has the biggest collection of managers with an impressive CV. But no one really cares about the old names anymore. There's nothing impressive about Wenger or Pellegrini. What's exciting for the league is the new guys coming in, Conte, Pep and Klopp. Pochettino stepping up. United should be careful that they don't end up with a manager who might not be as great anymore as he once was.