The DL/Westjet JV is merely cross border, isn’t it? So what westjet does transatlantic is probably not a consideration for DL.
Agreed on VS but my guess is that DL is assessing the transatlantic JV partner(s) that they swap the most passengers with, which would be AF and KLM.
DL and WS actually withdrew their application for a joint venture when the US Government told them they would have to give up slots at LGA to get it approved.
I’m a bit surprised that there is not some JV standard for premium economy service that the carriers have to meet. VS and WS are outliers serving drinks in glass cups — DL did when they introduced Premium Select but then switched to plastic due to a galley configuration issue I think.
The market for international premium economy is strange… pre-COVID it seemed to be replacing J in company travel policies, though it seems to have picked up some high-value leisure travelers. Airlines seem confused as to whether it’s a part of the economy cabin, e.g. LH recently eliminated complimentary spirits in both Economy and Premium Economy.