In a league where the league winner is more or less a forgone conclusion before the season even starts, barring a spectacular collapse (and even that’s not enough as Bayern showed you can recover from basically an awful half season and win the league still), exactly what metric are these managers supposed to be judged by if not by the only competition where they aren’t already expected to win by default? How else do you properly assess their performance? That’s not to say Allegri has underperformed as two CL finals in the last 5 years is a decent return, but of course the CL is going to be a big factor in assessing managers in these leagues if the other competitions are so uncompetitive.
Edit: One of my Barcelona supporting friends summed this up pretty nicely when he was talking about why Valverde underperformed this season. In a season where Madrid collapsed, and Atletico fell away, having not actually been any serious competition for the league in many years, how are you supposed to look at a league title won more or less by default with the strongest team in the league, and say that this manager has done anything impressive, when he’s failed so spectacularly in the CL two years in a row.
I agree with you about the league being a foregone conclusion, but it's not a reasonable expectation of any team, player, or manager to deliver the Champions League. Not least because you need luck.
There are countless examples of this, but, for example, the first two English teams to win the Champions League, Man United in 1999 and Liverpool in 2005, were both extremely lucky in the final. And in the semi-finals, the only goal Liverpool scored may not have gone over the line, and United were 3-1 down against Juventus, and if they hadn't scored an injury time equaliser in the first-leg then they would have been dead.
But that wouldn't have made Ferguson a worse manager, it's just the nature of a knockout competition, in which one bad period of 45 minutes can kill you. The best coach, Guardiola, hasn't won the Champions League for some time. The best player, Messi, hasn't won the Champions League for some time. It's not easy to win because you're always playing the top teams in the world, and any mistakes and you're finished. It's just not reasonable to say to any manager that you have to win the Champions League, or else you've failed.