Paul VI's constitution created a new missal but it did NOT authorize, permit or command any priest to use it, and by extension, for anyone to attend it. This is because to use this new missal is a violation of Quo Primum, which is still a valid law and which supersedes Paul VI's law. Paul VI could have, in theory, changed or revoked Quo Primum but he did not. So Quo Primum is still in effect and it allows any priest to say the tridentine rite, forever. More importantly, it COMMANDS, under pain of sin, ALL priests and faithful to ONLY use the tridentine missal. Many people forget that this law is both a permission and a COMMAND.
The point is, many people say that Quo Primum is infallible. I'm not sure that is accurate, since it is a law and legal matters aren't usually infallible, because that has to do with doctrine. But either way, Quo Primum is still a valid law and since it's permissions and penalties are still in force, then anyone who says or attends the novus ordo is commiting a grave sin of disobedience against the law of the Church.