Under the CRA, if a hardware product cannot run the latest software version, can the manufacturer stop offering any supported security-update path for that product?
No.
Recital 40 says that where a hardware product, such as a smartphone, is not compatible with the latest version of the operating system it was originally delivered with, the manufacturer should continue to provide security updates at least for the latest compatible version for the support period. So Article 13(10) does not let the manufacturer point users to an upgrade path that the hardware cannot actually use and then stop supporting the compatible branch.
recital 40