The traditionalists who think the NO is invalid, or doubt its invalidity, I think really are a small, but vocal minority in the traditionalist circles.
People who hold such views really are verging on Sedevacantism. The Church cannot promulgate an invalid Mass, which the majority of Catholics will celebrate. This would imply the gates of Hell have prevailed. Traditionalists and Catholics loyal to the Magisterium are obliged to believe the Novus Ordo has a valid consecration, and is therefore is a vaild Mass.
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