The quadruple was always highly unlikely. Too many things had to go our way to win it. Note that the players who played vs Dortmund struggled: Kevin injured himself, Dias and Rodri weren't great. It was a fantasy to hope that the team from Dortmund would turn up at Wembley and beat the shit out of Chelsea. We struggled even against West Ham the previous time we travelled in the CL and had less than 3 days to prepare.
Let me point out three major problems which have stopped us.
First, Aguero has been barely available all season. To play all season with just one genuine striker (Jesus, and he missed 2 months as well) is a bit too much even for an excellent squad like ours and a great manager like Pep.
Second, Sterling hasn't been in form this season and has fallen off a cliff over the last 6-7 weeks. So, we are missing not only Aguero's, but also Sterling's contributions in the run-in: that is, the goals and assists of our leading goalscorers from the previous 3-4 seasons. That we hoped to win everything without both of them contributing goals in the run-in was a bit naive in hindsight.
Third: Kevin's injuries. We managed to do well without him in February, but the likes of Bernardo and Gundogan played a bit too much and Pep has to be careful with their playing time in the most important part of the season. It didn't help that they started all 3 games during the international break.
Besides, Jesus has had a start-stop season. Plays his best game vs Leicester, but is on the bench vs Dortmund, which, let's face it, has probably disillusioned him to some extent. If he can't start a big game when Aguero is out, has he a good future at the club at all? Would we have been worse off with him on the left and Foden on the right?
Given our problems, especially Kevin's injury, we'll do well to win the PL and very well to win also the league cup. If we managed to win the PL + the CL without Sergio and Kevin, and with Sterling in a very poor form, that would be Pep's best achievement ever. In any case, Pep and the lads will try to do their best.