hard drive - SSD cache in SSHD compromises the lifecycle compared to standard HDD?

The limited writings and capacity\price of the SSD are known drawbacks as the sensitivity to shocks of the HDD.

The hybrid SSHD (standard HDD+SSD cache) have a classic HD combined with a small SSD used as cache and managed by the firmware of the SSHD.

Now I want to know what happens when the SSD cache reaches the writing limit quote for all the cells, the two possible alternatives are:

a) simply the firmware stop to use the SSD caching and the SSHD becomes a standard HDD

b) the SSHD becomes unusable

What is the right answer? (The b alternative would make the SSHD choice, the less durable alternative and the worst choice ever for a server.)

I have searched reliable sources about this, but I haven't found anything.

The mechanical problems of HDD are very rare if they they are not subjected to shocks during the read\write operations, in standard condition the MTF range between 1 million to 1.5 million hours for a modern HDD.
In the SSD, particularly SSD TLC, wearing is a problem, MLC typical maximum PE-cycle-per-block numbers range from 1500 to 10,000 (5000 mostly). Reach 5000 cycles is relatively easy if the storage is used intensively (especially in server use). So It's really important the SSD cache durability, and in addition must be considered also that cache-SSD sectors in SSHD are used even most intensively of a standard SSD that can distribute the wearing on a larger space.