According to their staff as recently as 2020, sales of core books – and even supplements – were increasing over time instead of declining. And, new supplements were usually selling better than older supplements. According to staff in 2017, the 5E PHB had sold over 800,000 copies. That number is now well over a million, in all likelihood.
This success was a surprise to WotC, who had planned 5E’s revenue to come primarily from non-book sources.