That's a conservative estimation of the lifespan (how hard you can hit it while maintaining a very low probability of SSD failure). The more interesting question is "How likely after n years is the SSD to be the first non-replaceable component to fail on this laptop?" I don't know the answer to that question, but I'm guessing it's a good long while before the answer to it goes above 0.5.

Apple have been shipping laptops with non-replaceable SSDs since around 2017. Anecdotally, we hear so much worrying about potential future SSD failure, and yet so few people saying things like "I bought an M1 Air two years ago and now the SSD has died".

SLC makes sense for almost zero use cases, even in a data center. It's simply the wrong tradeoff between capacity and performance. It's not enshittification that you can now have a cheap multi-TB SSD to hold a large collection of movies and games and still have an SLC cache for the small portion of your data that isn't mostly static.