With any digital camera sensor replacement is a question of how many spare sensor boards the manufacturer has in stock. Manufacturers do this for in-warranty repairs. Out-of-warranty sensor board replacements are usually prohibitively expensive. If your life really depends on having your M8 sensor serviceable for significantly less than the cost of a new used M8 if the sensor does break for some unknown reason, I wouldn't buy it.
In practice, however, the sensors in oldish digital cameras work just fine. Just stop worrying about it.
Bing! You're hypnotized!