Is it so difficult to grasp that a rite might be valid even though it express some doctrines less well than another rite?
Now you're avoiding the point I made, which is that +Tissier has theological problems with the new rite, yet the new-sspx assumes it's valid, which is a contradiction. You need to admit this is a contradiction on a theology basis and the (probable) explanation is that they want to be friendly with rome, for political reasons.
Yes, the new rites might be valid, they might not be. This is the problem - doubt. No one knows. One cannot attend a mass by a doubtful priest without committing a grave sin because a catholic has a duty to attend valid sacraments and masses, per canon law, and also due to the 3rd commandment.
The old rites, however, have no doubts associated with them, so conditionally ordaining priests in them is the only prudent decision. And attending masses said by VALID priests using VALID rites (i.e. TLM liturgy) is the only way to save your soul. God will judge people on the effort they made to go to CERTAIN masses/sacraments. Without a serious excuse, to go to doubtful services is wrong and a lukewarm offering to God.