Crisis in the Church -- an analogy
It's often useful to use a parable or analogy, to help us break outside the "boxes" we've created in our minds for the various groups.
Here is some food for thought:
For our hypothetical example, let's take the Society of St. Pius X. For purposes of this discussion, let's presume the priests are valid, their Masses are valid/pleasing to God, etc and that much good is currently being done.
Now let's say something happens. Say a premature "deal" was worked out with Rome, and a few years later many (but not all) of the SSPX priests started getting worldly, as wealth, human respect, etc. started going to their heads.
Many priests start using a more "up to date" Bible translation for readings. Do they still have a valid Mass?
Many priests start downplaying the need for architecture, ornamental decor, beautiful art, etc. in favor of bigger buildings which hold more people. Do they still have a valid Mass?
Some priests ditch the cassock. Do they still have a valid Mass?
Some priests even start wearing shorts and a T-shirt. Do they still have a valid Mass?
Some priests scale back their spiritual duties: breviary, daily meditation, daily Rosary, daily Scripture reading and/or daily study. Do they still have a valid Mass?
Some priests start watching TV. Do they still have a valid Mass?
Some priests start disliking their vocation; even saying Mass. They begin rushing through the Mass. DTSHAVM?
Some priests start joking more during sermons. DTSHAVM?
Some priests start avoiding "hard topics" that might offend the parishioners. DTSHAVM?
Some priests start getting addicted to comfort, higher-end food, human respect, etc. DTSHAVM?
Some priests start allowing different kinds of hymns during Liturgical services. Solos, opera, different instruments, protestant hymns, etc. to better please people. DTSHAVM?
A few priests even commit various scandals against the Sixth and/or Seventh commandments. DTSHAVM?
A few priests believe and/or promote things that are offensive to pious ears (e.g., "Fatima is not a true apparition"). DTSHAVM?
A few priests begin to espouse doubtful or even heretical positions. A) Do THESE priests still have a valid Mass, and B) Do SSPX priests in general still have a valid Mass?
...
And on down the line. At each stage, it becomes less "pleasant", but WHEN exactly does the Mass start becoming INVALID just because they've become ICKY (for lack of a better term)?
And if someone walked in FIFTEEN YEARS FROM NOW on the whole subtotal of these changes, they'd say "yuck!" and could easily say, "That's no Mass. Jesus isn't present there!" and their opinion would seem to be quite believable.
But when you broke it down, you'd see there wasn't any substance to the argument. The priests were validly ordained before; they still are. They said the proper words of consecration before, and still do. They used pure wheat flour with no additives for the hosts before, and still do. They used grape wine before, and still do.
Those who believe the Novus Ordo is invalid should be able to say exactly WHY -- and "Ick!" isn't a good enough reason.