All things therefore whatsoever they shall say to you, observe and do: but according to their works do ye not; for they say, and do not.
This applies to popes that are evil, e.g. a hypocrite of a pope that would correctly preach chastity but while having concubines for himself. The problem with the post-Vatican II popes is that what they PREACH is in itself wrong, so that we may not do and observe what they say to us.
The point is how Our Lord dealt with the bad leaders of Israel at the time and the wicked authority. Instead of endorsing a deposing of the High Priest and declaring the Seat of Moses vacant, Our Lord did not not usurp the lawful authority of the very high priest who had Him put to death.
There is lesson for the sedevacantists here: If Our Lord counselled the Jews to observe the teaching of the Scribes and the Pharisees, then the sedes that think they can just deny and disobey lawful authority contradict the teachings of Christ. They share the same error of private judgment that ensnared the Jansenists and the Protestants in which they do not hear the Church but themselves.
The thing is that the Pharisees were still orthodox in their teaching. Do you think that if the Pharisees had said to worship idols that Christ would still have said to the people to obey their teaching?
The Pharisees were extremely orthodox in the law, it's just that they were proud hypocrites when it came to practising the law. The modernists are not orthodox in regards to the law at all, which is why the analogy is a false one.
An excellent distinction...one that needs to be stressed over and over again.
"For there is not any thing secret that shall not be made manifest, nor hidden, that shall not be known and come abroad."- Luke 8:17