Worth noting that the ironing board back isn't completely flat, it does have curves at the edges. But I'd rather a flat back than one shaped to curve my back unnaturally.
Having had DVT twice and being tall, it's not one I am willing to pay. I don't like Fainsa Sophias very much, but what I really value in 80x is that I fit in every Standard seat - on Pendolinos I fit only priority and table seats.
To me airlines have this one right - invest in the base as that's 99% of comfort, but have a thin back to increase legroom (or even if you don't, get more seats in). Some of the most recent designs, which are no doubt very expensive, are excellent, such as the new easyJet ones.
Indeed, as
@zwk500 pointed out, it's skimping on cost that is the main issue (as I'd be more than happy if every train in the UK had Grammer's finest). If you want a quality, comfortable and innovative seat design, you have to cough up. Fainsa seem to trade mostly on "seating on the cheap", and the UK railway takes it a further step down by specifying regional seats on long distance trains for which they weren't designed. The Sophia is being used on the South Wales trams, and that's what they are designed for - basically journeys of under an hour.