Premium economy is actually an interesting one. It wasn't a thing a decade or two ago, and now all airlines seem to be clambering to install the product.
One big problem for me is that different airlines offer very different things. For some airlines, premium economy is economy with a bit more legroom. For other airlines, it's much more of a business light product. BA's product isn't the worst, but it is closer to economy plus than business light. Virgin Atlantic, on the other hand, is more like business light.
A lot of the growth for premium economy comes down to two things.
- Holiday makers who wouldn't consider business class, but can afford to 'treat themselves' to premium economy.
- Corporate policies which forbid business class travel, but allow economy and, by extension, premium economy. It sounds wild, but this is actually not too uncommon.
I've only personally flown it a few times. PE tends to be priced much closer to business than economy which makes it not worthwhile if I am going to pay myself. Less premium Asian airlines (Vietnam/ the mainland Chinese airlines etc.) tend to price their business cheaper than BA premium economy on routes I fly. Had several cracking deals on Aeroflot back in the 2010s where they were cheaper than BA economy. Aeroflot's premium economy was definitely more like business light as well.
Advertised as economy, however is better than your 'usual' economy. (3-3-3 instead of 3-4-3, fairly generous seat pitch)
10 abreast on the A350 would be unusual. When it was initially launched, it wasn't possible at all. Though it is now possible to put 17 inch seats in as Airbus slightly redesigned the internal fascia panels after entry into service to add 5 inches to the cabin width to allow said 17 inch seats.
17 inches on a 18 hours flight from Sydney to London would be pretty harsh. Especially as it would certainly be priced as a premium product.
Note planes do vary in width.
3-4-3 on a 747 is pretty comfortable as the plane is pretty wide.
3-4-3 is one of my bugbears for people who just love Emirates or Qatar. They were among the first to introduce it on the 777 and it certainly makes for a cramped cabin.
Similarly the 787 was originally though of as a 2-4-2 plane, but only launch operator ANA use 2-4-2. All other airlines installed 3-3-3 which is similar to 3-4-3 on the 777.