Prior to the pandemic, many flights were loaded at less than 100% loads - mainly ex-East Coast. I believe that the late ORD-LHR was loaded at 60% in CW at one time. I don't ever remember having any (significant) issues.
What's new is the reduced WTP loading - although I feel like I'm increasingly seeing fewer people eat in WTP on such services. Take JFK - many WTP customers are Gold/Silver, thus they have access to a decent lounge with decent drinks and eat before they get onboard. They also may have access to the Arrivals Lounge where they can have a proper breakfast, rather than some manky croissant that one of us has nuked in the ovens since we came off first break.
As always the execution is key, and will be interesting to see how they do it. The last thing we need is to be 10 meals short in WTP and CW.
I'm under the impression that a lot of work is being done with regard to pre-ordering, which is why it's taking a little longer than one may think. It will be a different system from that used previously.