I believe that when the Indult was given in the 1960s, the reason given by the bishops as I read the document was so that people could, actually, ‘do even more’. Apparently the thinking was that since meat was no longer the ‘luxury’ it had been (hard to realize, but in the 1960s meats were cheap. I mean, really cheap), and since even back then things like lobster (originally itself a very, very cheap foodstuff!) and other fish dishes were actually considered ‘more luxurious", and with people also becoming vegetarian, and since we were trying to ‘blend in more with the culture’ (and the Catholics ‘eating fish on Friday’ were a very visible sign of ‘culture’ and some called it ‘ghettoization’), the bishops had come to believe that (in the typically ambiguous language of the time which alas exists to this day) it would be a wonderful opportunity for American Catholics to be able to find their own ways to have a more meaningful relationship with God (the word penance being ‘played down’ to the point of almost disappearance.) You really have to look and extrapolate to get the idea that the whole reason for removing "abstinence’ was to encourage people to deeper penance as a part of their long-standing and traditional deepening of their faith and relationship with God. The three part, "prayer, alms, and fasting/penance’ has always been there (Jesus Himself remarked that 'when the bridegroom has gone his friends will fast–and fasting is a penitential action), and it was meant to remain. . .
BUT (you knew there’d be a but) this is couched in such ambiguous terms, and the catechesis was so abysmal, that it never came across correctly. (Just as happened with the many documents of Vatican II).
And so one can say, quite correctly, that it is never explicitly spelled out as in “Thou shalt do penance on every Friday, whether it be abstinence or something else”, and therefore, one need not do penance (though still abstain from meat on Fridays in Lent unless one is released by the bishop, or one falls into one of the many categories of exempt, or one forgets or doesn’t wish to cause offense). . .
And one can also say quite correctly that even if it is never said in KJV ‘the words of the Lord beeth written in ye red’), since the norm of the Church remains year-round Friday abstinence and since the intention of the bishops was clearly that some sort of ‘replacement penance’ would remain on a Friday (they probably thought since we were so smart in asking to do more, we didn’t need to be TOLD to), that we should indeed do penance every Friday.
It’s just another of the many, many ‘ambiguities’ in which you can find two perfectly ordinary, pleasant Catholics who truly desire to live their faith, honor God, and obey His teachings, come up with polar opposite understandings of those teachings, and who totally, utterly, sincerely believe their way is the right way and that the other person is the spawn of the devil. They can each quote Scripture to support their position, bring out Church documents, bring out Church leaders, saints, popes to ‘support the position’, and even use the same dang documents because they’re so ambiguously put!