I really hope each and every diamond is emailing or calling Delta to ask them to change their policy to allow you to upgrade into premium-economy with a global upgrade certificate and then waitlist for business-class, if it opens up. To not allow this is just silly, but Delta will only change the policy if enough of us complain.
Of course, it's obvious that Delta wants all of us to just buy a premium-economy ticket. I'll withhold judgment until someone actually flies it for 14 hours, but it's disappointing that the premium-economy seats are only one-half of an inch wider than an economy-class seat. It's also disappointing that Delta went with four seats in the middle section of premium-economy instead of three seats, which is the premium-economy configuration of many other airlines flying the A350. It could have at least separated the four middle seats into two pairs, giving some privacy.