More precisely, endurance and retention become exponentially lower with each additional bit stored per cell, while capacity only increases multiplicatively.
They are better. SLC is improving as much as MLC. The ratio of speed, durability, and capacity is the same between SLC/MLC/TLC, but modern MLC is faster and more durable than 5-year-old SLC.
You are probably writing this from a computer using a TLC SSD. Outside of applications that need extreme latency, pure SLC has almost completely disappeared from the storage world. From materials science to management algorithms, a lot has advanced in flash technology in terms of durability.
It's true that TLC and MLC have for good reason displaced SLC. However, in no way are they even anywhere near the old SLC in durability. ~5 years old SLC literally had 100 times more write endurance (as in, how many times you can rewrite each bit, not total amount of writes in the drive) than typical modern TLC.
That is pretty much true, and it's pretty much the only stat that hasn't improved. However, write endurance isn't really a factor in terms of data durability. It has to do more with the drive's ability to continue being written than with the safety of data that has already been written. If your drive goes read-only or loses capacity, that has to do with write endurance. Neither of those involve data loss.