Yes lonely you're right, but the only scenario where "waning at the same time" is relevant is against a resurgence of a closely related virus not a new one, new ones obviously are not protected by past infection.
If there's a new variant which defeats existing immunity then it's irrelevant if people today were infected in 1 month of 4 months, the immunity is invaded.
So the only situation where waning immunity is relevant is if the same or very similar variant arrives such that waning immunity is relevant, and that is where, as with all other respiratory viruses, it won't be that everyone has waned at the same time, it will be enough people have waned and the conditions are such that R swings above 1 to trigger the wave (seasonality, even if it's just more people being indoors) Simply individuals waning is not enough to trigger a wave as there would not be enough candidates.