I listened to many of Mons. Hesse´s speeches and it is always the same story.
He makes the claim that some major parts of the Mass belong to the depositum fidei , but where does he get that from? As all the Sacraments were instituted by Our Lord, the liturgies are in substance of divine origin and cannot be changed by the Supreme Pontiff in this regard ("...nullam habet potestatem mutandi"). But the biggest part of the ceremonies surrounding the Holy Sacrifice itself are of ecclesiastical origin and are therefore subject to the ecclesiastical lawgiver, as one can easily read in any book of Canon Law.
Anywhere I read, the liturgical laws are named in one sentence with disciplinary laws, which are infallible (theologically certain
, the manuals tell us). On the other side, I cannot find any trace of Mons. Hesse´s claims.
I copied some relevant pages from Wernz-Vidal "Ius Canonicum", Tom. IVa "De Rebus", tit. IX., "De Legislatoribus et Liblis Liturgicis", pag. 410-418):http://imageshack.us/g/845/42978524.jpg/
St. Pius X. changes were the most extreme changes to the liturgy since St. Pius V. He completely changed the Psalter which was in use since the times of the Desert Fathers. This was a complete innovation and novelty.
(Of course, there is at least one sede priest who rejects Pius X. for precisely this reason...)
PS: I don´t know why I said Urban VI., of course I meant Urban VIII. who, sadly so, changed the beautiful hymns of the Roman Breviary. Luckily, they are still to be found in the Monastic ones.