With So Many Planets, Isn't an Earth Bound to Occur Somewhere?
The universe contains an enormous number of stars and planets. Even if the probability of producing a highly habitable planet around any particular star were small, a sufficiently large number of opportunities might make planets like Earth unsurprising.
On this view, Earth does not require purposeful arrangement. It may simply be one successful result among a vast number of planetary experiments.
The more planets nature produces, the more opportunities there are for an unusual combination of favorable conditions to occur somewhere.
Why This Matters
Rarity alone does not establish design. An event with a small probability can still occur when there are an enormous number of opportunities for it to happen.
Any argument based upon the apparent rarity of Earth must therefore consider not only how demanding the requirements for habitability may be, but also how many planets and planetary histories exist.
The number of opportunities matters when evaluating whether an outcome should be regarded as surprising.
Response
The enormous number of planets is relevant and should not be ignored.
But the argument cannot be settled merely by comparing a vaguely described low probability with a vaguely described large number of planets. We presently do not know the probability that a planet will develop long-term habitability, originate life, or sustain complex life.
Nor do we yet know how independently the relevant planetary conditions should be counted. Planetary characteristics develop together through formation and evolution.
For now, the number of planets provides an important possible explanation for apparently rare outcomes, but the actual frequency of inhabited worlds remains an observational question.
Question That Remains
How frequently do planetary systems actually produce long-lived habitable environments, and how frequently does life arise within them?