So Group Stations aren't actually being phased out, just that the groups will be written differently on tickets?
Some people within the industry have been trying to phase out these tickets for years, but there is no way to completely eliminate them without eroding passenger rights and flexibilities.
Here is an example of the sort of thread that these people like to use against passenger rights:
https://www.railforums.co.uk/threads/epsom-to-euston-aka-london-terminals.185005/
Their argument goes that it's confusing, so must be abolished. The problem is, any replacement is going to be detrimental to passengers.
There was, for many years, a direct train from Richmond to St Pancras. This now lives on as London Overground, albeit with a change of trains required at Willesden Jn to get to Euston. Certain people within the industry are very keen to remove this route as being valid for tickets from London Terminals to Richmond (and beyond) and abolishing these groups would be the perfect excuse for them to do this.
Next time GWR get diverted into Marylebone/Waterloo, expect major ticketing related problems, to give another example.
How incredibly passenger-friendly
You are quite right to see through this; some people would love to make Richmond to London Terminals tickets issued as only valid to Waterloo/Victoria (and, if we are lucky, Charing Cross, London Bridge etc would be retained) and force people going to Euston to use PAYG.
They also want to stop people going from Birmingham to Paddington, Peterborough to Liverpool Street, etc etc...