I don't think anyone is falling over themselves with unmitigated glee at the thought of a free middle seat. But I don't like speaking for others very often. So speaking for myself, what I can say is that I currently don't appreciate that SAS fills every seat in Plus when their major competitors don't in their premium cabin - and for very similar price points (LH business is my main example).

As a tall and broad person, I would absolutely prefer more legroom at the pointy end of the plane. But if I had to choose between a guaranteed free seat next to me and a few more centimeters of legroom, I will always choose the free seat. No fighting over the armrest, space to use the free tray table and seat, etc. I value that gap between me and the next person. I can deal with only having standard legroom because the European airlines I fly haven't yet packed economy like sardines by reducing legroom/seat pitch to make space for premium seats (especially UA which I had to let my employer know recently that I am literally too tall for their standard economy seating).

For me, given the above, I also appreciate that the airlines can flex the number of premium seats as needed because they aren't constrained by seat design. If they need to a few more rows, they can add them. So long as that means I get to travel with fewer people in the row, I can live with it.

And finally, I think the other trade off is that we largely avoid the fight for upgrades on short haul. Upgrades have become somewhat of a blood sport in recent years in the US3, pitting frequent flyers vs frequent buyers. Only to be placed behind a growing number of people paying dynamic pricing for cash upgrades. But the seat is awesome so I understand it. Just the same, I am glad we largely avoid that in Europe. The seats just aren't worth fighting for in that way.

But as it's the internet, YMMV on what you value and prioritise. And that's absolutely fine.