I understand that, for many, giving up meat is no real penance - especially if you choose lobster as a substitute! But for many - I’d say even a majority - of people in the West, at least, where meat is regularly eaten, having to forego meat does involve a conscious decision, even if the substitute meal is pleasant.
As I see it, leaving the choice of penance up to the individual requires a great deal of discipline and maturity - it is human nature to be inconsistent, to chose a penance that would not inconvenience one too much. Example: we normally abstain from meat on a Friday, but sometimes one has a craving for meat, or one is invited to a function and those cocktail snacks look so tasty, etc, and meat is eaten, and then one says, “okay, I must give something else up - well, I haven’t had a glass of wine today and don’t really feel like it, so that’s what I’ll give up”. In South Africa, you can even substitute “other works of piety or charity” - without saying what suffices. So…let’s just say an extra decade of the rosary, a few extra prayers, the duty is done.
The result is that, not being bound to anything in particular, it is very subjective, and also…the whole Catholic IDENTITY has also been diluted. It was something all practising Catholics did - now that is gone, and most people don’t seem to realise they are bound to anything.
God bless
Mitch