I imagine that each MOSFET is designed to sustain a certain amount of switching (for instance 10^9 times)
You imagine wrong then.
There are several mechanisms by which a MOSFET might be damaged. Any particular type of MOSFET is given ratings so that if the designer stays within them, the lifetime will be essentially unlimited, certainly exceeding the expected lifetime of any equipment they are built into.
Switching is a high-stress moment for a MOSFET, resulting in high channel dissipation as there's a current flowing with a voltage across the channel. However, the SOA (safe operating area) rating specifies the maximum voltage and current for any given time, and heat within this limit is absorbed safely and without exceeding the maximum junction temperature.
MOSFETs are made by diffusion at high temperature, and they can be unmade by the same process. The rate of diffusion increases exponentially with temperature. Most devices have a rated maximum junction temperature of 150C. This temperature is so far below the temperature at which diffusion happens rapidly that it would take 'a very long time indeed' for the structure to diffuse into something else.
This is in contrast to a switch or relay, whose contacts suffer from arcing and erosion on every switching operation, and so do have a lifetime rated in the number of switching operations before failure. 10^9 is a rather small number for switching events in a transistor, but quite a big number for switching events in a relay or switch.
You mention in comments that you are concerned by UTE C 80-810, which has the following comments under power transistors >5 watts, page 43 if you're looking at the 2000 version.
The cycles referred to are thermal cycles, cold/hot/cold when switching equipment on and off. This subjects the package, its interface to the die, and seals to the leads, to mechanical stress, which can cause failure.