A very interesting thread and a lot of interesting points being made.
The truth is, for the 'average Joe', the 80x represents a step up in luggage capacity, with much better capacity at seats both in the racks above and at floor level. I travel with a rucksack and used to stow it above my head on HSTs to provide sufficient arm and leg room, but find on the IETs it fits neatly beside me sort of half underneath the seat.
Where the challenge comes in is around 'bulk' luggage. There's no doubt the cycle spaces are more awkward and that the end of coach luggage 'stacks' are smaller than on HSTs. A power car van or TGS type solution just isn't favourable these days - passengers being very weary of leaving their items even half a coach length away, let alone potentially several coaches away. Luggage stacked against doorways on the HSTs was an endless problem, even with luggage cage operations in place between Paddington and Plymouth or Truro - it could be quite a challenge to persuade passengers to part with their belongings.
A key difference between the luggage of 1976 and that of 2020 is the value of it, which is why rolling stock manufacturers try to put as much luggage space in adjacent to seats as they can. Folk are usually travelling with phones, tablets, laptops - other expensive items they couldn't possibly do without for a few days - which is why most modern designs centre on maximum overhead rack capacity.
Let's be frank; surfboards represent a tiny, tiny proportion of even the leisure market likely to use GWR, so it's questionable if an entire train fleet should be modified to take into account this specific. Where does this tailoring end? Should trains be designed so I can take my motorbike aboard as that's a popular summer activity in the West Country? The HST "luggage" space in the TGS and Power Car vans were actually designed around the Red Star Parcels market, not with excess luggage in mind. Let me tell you having worked many HSTs over the years, piling the TGS full of luggage really isn't a good solution - try dispatching, using the guard's office or accessing the emergency equipment when there's suitcase piled to the rafters.
I believe LNER, with work shared with fellow IEP operator GWR, is doing the right thing in regards to adding the extra luggage stacks in place of the window-less seats. It'll be interesting to see if GWR now follows suit. That said, there's nothing stopping any operator using the solution of putting a cover over those seats and using it for luggage storage in any case.
All that said, there's a clear swing away from the commuter market to the leisure market, so it'll be interesting to see how this affects the interior configuration of all rolling stock in the future.