Millions of insterchangeable lens cameras were sold, millions of lenses for dSLR were sold. About 160 Million interchangeable lens cameras sold 2007-2021 alone, per CIPA
https://nikonrumors.com …r-2022-cipa-numbers.aspx/
IOW there is an inherent demand for used lenses for dSLR. And everyone is not stampeding to changing over.
Mirrorless camera volume has been rather stubbornly limited in growth of Units, stuck at a plateau for many years around 3-4 Million units each year, and gaining market share as a Percentage only due to the decline of dSLR sales. 2021 Unit volume for mirrorless was about the same as it was in 2012. We have not yet seen CIPA numbers for 2022, to see if mirrorless has finally been able to get above its longtime plateau.
Many dSLR owners are resisting changeover, in part waiting for mirrorless designs to reach personal threshholds in features/performance and in part due to the inherent expense of purchasing mirrorless accessory items that are not yet plentiful enough on the used market. That RF lenses are typically more costly than 'the same' lens EF equivalent does not help stimulate changeover, especially when EF lenses can be put on RF bodies...keeping them in demand even for mirrorless body purchasers.