Shorter lifetimes means more renewal events, which means more individual occasions in which LE (or whatever other cert authority) simply must be available before sites start falling off the internet for lack of ability to renew in time.

We're not quite there yet, but the logical progression of shorter and shorter certificate lifetimes to obviate the problems related to revocation lists would suggest that we eventually end up in a place where the major ACME CAs join the list of heavily-centralized companies which are dependencies of "the internet", alongside AWS, Cloudflare, and friends. With cert lifetimes measured in years or months, the CA can have a bad day and as long as you didn't wait until the last possible minute to renew, you're unimpacted. With cert lifetimes trending towards days or less, now your CA really does need institutionally important levels of high availability.

Its less that LE becomes more of a single point of failure than it is that the concept of ACME CAs in general join the list of critically available things required to keep a site online.