The present restrictions, in Rome, practically, are not there only for the Latin Mass but also for Mass in Italian. I could cite specific examples when conservative Catholic priests were called up when they interpreted the baptism of desire and invincible ignorance without the common false premise, which is politically approved.

 What are their arguments for questioning Benedict XVI’s document?

They are laid out clearly in the accompanying letter. They can also be found on the blog of Andrea Grillo, a lay professor of liturgy at St. Anselm's who has been extremely hostile to Summorum Pontificum. His idea, taken up by the Pope and the crafters of the recent motu proprio, is that the traditional Mass represents a state of doctrine prior to Vatican II while the new Mass represents the doctrine of Vatican II - something we all already knew. Therefore, it was no longer necessary for the traditional Mass to be a right, but only a tolerance, and even then a tolerance only granted to faithful and priests to help them gradually transition to the new Mass.(Emphasis added )

So the main reason is doctrinal?

Yes, and it is very important to say this and to be aware of it because, paradoxically, this is all very providential. It is of course very painful. It will hinder the diffusion of the traditional Mass. It will start new persecutions. But, on the other hand, it puts the finger on what hurts, namely the doctrinal status of Vatican II, which has never been settled.(Emphasis added )

So, the bottom line is that this hatred of the traditional Mass has a doctrinal basis?

Absolutely. It is the hatred of the Tridentine ecclesiology, of all that this Mass represents from the point of view of Eucharistic doctrine as well as the doctrine of the Church.- Italian Bishops and Cardinals were the origin and moving force behind Traditionis Custodes (an Interview with Fr. Claude Barthe) (Emphasis added )

https://rorate-caeli.blogspot.com/2021/07/italian-bishops-and-cardinals-were.html#more

 His idea, taken up by the Pope and the crafters of the recent motu proprio, is that the traditional Mass represents a state of doctrine prior to Vatican II while the new Mass represents the doctrine of Vatican II - something we all already knew. 

Lionel 

There is a misunderstanding. The liturgy is not responsible for the Tridentine ecclesiology. Since even at the Novus Ordo Mass we can interpret Vatican Council II without confusing LG 8, LG 14, LG 16, UR 3, etc as referring to physically visible cases instead of physically invisible cases. If it is the latter then there are no  practical exceptions in 1965-2021 to the past Tridentine exclusivist ecclesiology of the Church.

So the present restrictions, in Rome, practically, are not there only for the Latin Mass but also for Mass in Italian. I could cite specific examples when conservative Catholic priests were called up when they interpreted the baptism of desire and invincible ignorance without the common false premise, which is politically approved.-Lionel Andrades