There is a lot of frustration from people that are playing retail who see their raids completely cancelled and all their friends or guildmates playing Classic, particularly those that were not interested in playing Classic from the start. And I guess they fear a lot of people will never come back, which will jeopardize retail to some degree, but most certainly their guild and progression.
However, we really have to consider that BfA is near the end of the cycle content-wise. We will have a better appreciation when the expansion is released of what people do, but what everyone seems to forget here is that overall, the number of WoW subscriber has increased dramatically, which can only be good for everyone, both Classic and Retail.