This was roughly my thinking. From the article:
“The High Court agreed with us that Government acted unlawfully. It is vitally important that fairness for songwriters, composers and performers is written into the law,” UK Music CEO Jo Dipple commented on the ruling.
The moral argument being made here is fairness for the artists/composers/authors. They can't benefit after their death so such an argument does raise the question of why the copyright term extends so far beyond their death: 70 or 90 years is excessive. Extending a single generation - 25 years - after death seems sufficiently respectful recognition of the work done.