Prem Eco is a good product and popular with the pax, but according to my employer doesn't add much to the bottom line as the price premium is quite low compared to the loss of seats. But, it looks good in the marketing. Don't know about Norwegian, but there is a lot of 'in the checkin queue' sales as upgrades, which is a healthy uplift to an economy ticket. If it was such a good thing, there'd be an airline out there operating a Eco Plus cabins.