To be fair, Fr. Hesse says that he sides against those who side with the old theologians. When he says this he seems to indicate that he could be wrong and that the matter is not decided. He makes clear arguments for why he took that position and also says that the purpose of the law book is not to declare if it is possible but only to speak about the evil or illicitness of the act.
It is never safe to go against the consensus of the approved theologians of the Church, especially today when the Faith and Holy Tradition are attacked on all fronts.
I don't think that this would apply to this particular case because no article of the Faith is at risk. It seems that one can have one opinion or the other, just as is the case with the belief in the Mediatrix of All Grace.
If we take what you have stated here as a reason, it would seem to be best to take the most precautious route, which is what Fr. Hesse asserts that he is doing. His point for discussing the matter deals with the New Mass, and it is his opinion that leads him to the very safe conclusion, SAFEST, that the New Mass in the vernacular is probably invalid (for all, etc.).
It is
never safe to go against the consensus of the theologians, whether the route taken seems to us more safe or less than the teaching they explain.
Actually, the
safest conclusion regarding the NO in the vernacular is that it is doubtful, not that it is invalid, because doubt can be established by the significant change in the words, but invalidity has to be certainly proven by proving that the words "for all" absolutely and in every circuмstance substantially change the
meaning of the form, which has not yet been done, as far as I know.
Fr. Hesse was an "original" theologian, which is to say that he made up his own novel theories, e.g. that VII was not an ecuмenical council because it did not infallibly define or condemn anything, and he gave a very original explanation why, which you can listen to online.
However, he can find no support for his definition of an ecuмenical council, and his theory is actually refuted by the fact that there was an ecuмenical council (and recognized as such by the Church) which was merely disciplinary and which did not define any dogma nor condemn any heresy - the First Lateran Council.