But actually Pius XII. himself says that it would still be better to follow the old law - which is actually quite ancient, I think.
In the Motu Proprio
Sacram Communionem [19 March 1957; A.A.S., vol. xlix., pp. 177-178] Pope Pius XII wrote:
But we earnestly exhort priests and faithful who are able to do so, to preserve the venerable and ancient form of the Eucharistic fast before Mass or Holy Communion.
Finally, all who enjoy these concessions are to endeavor seriously to compensate for the benefits received by becoming illustrious examples of the Christian life, especially by works of penance and charity (Omnes denique, qui his facultatibus perfruentur, collatum beneficium pro viribus rependere satagant fulgentioribus christianae vitae exemplis, praesertim poenitentiae et caritatis operibus).
This latter point seems to be forgotten nowadays. Perhaps it is because some Catholics neglect this grave obligation that they do not seem to derive much fruit from their Communions, and deliver themselves over to tepidity and laxity. This may be evinced by the paucity of the faithful who remain in their pews after Holy Mass for an appropriate thanksgiving for having received Holy Communion: something which was a problems long before the 1960's, as Rev. Fr. Garrigou-Lagrange noted in a chapter in his celebrated work
The Three Ages of the Interior Life. Perhaps this may be the explanation for the unnervingly high tolerance [or, perhaps, preference] for mediocrity in some traditional Catholics (whether sedevacantists, non-sedevacantists, none of the above, etc.); and perhaps why the whole mess in the 1960's and beyond happened.
The English translation of the Motu Proprio was taken from
The Pope Speaks: Addresses and Publications of the Holy Father (Vol. 4, no. 1, Summer 1957, pp. 7-8).