I don't think keeping running first like it was was an option. A first class seat as built takes up 50% more space than a standard one, so expecting a table to yourself in addition is taking the capacity to maybe half. If they aren't consistently able to get reasonable loadings at double price therefore then they'll progressively convert the space to standard.Exactly this -those travelling on a full first class ticket may find that table to themselves somewhat harder to come by now!
(appreciate that a table to yourself is not guaranteed in first, but a lot of people will pay the extra to increase their chances of getting one)
The fact that first was hugely overprovided on West coast, and people have been able to get underpriced upgrades or half carriages to themselves was never going to continue. Standard Premium is arguably the least bad response, given that the alternative was probably to convert more carriages to normal standard