There's a countervailing effect where the more often you do something, the better you get at it. You're right that the absolute number of errors will certainly

There's a countervailing effect where the more often you do something, the better you get at it. You're right that the absolute number of errors will certainly rise, but the fraction of attempts which have errors will likely fall. As legacy certs expire, the aggregate quality of certs will likely be higher. A secondary question is whether the gain in security is worth the required effort. Obviously Apple believes this, and LetsEncrypt is pretty easy, so even for hobbyists, it's probably at worst an annoyance.