that's what everyone said about MLC when it was first beginning life. which, at the start, it was slow, but as controllers matured, they became faster and faster and were outperforming most SLC drives. now they make server MLC drives now.
Top of the line MLC drives are outperforming generations old SLC. However SLC is still a lot faster for the same generation and we still have SLC drives out there.
Also MLC is 100% increase in data while same transistor cost while TLC is a 50% increase in data coupled to a 20% extra transistor cost giving you a net gain of only 25% net gain in data/transistor aka a cost saving of a mere 20%.
Yet the penalties in speed are supposed to be even greater than the SLC to MLC transition..
20% cost reduction is barely worth the speed penalty (write limit/lifespan penalty is indeed meaningless for anything but a high volume server)