In general you're right, but I believe that there are ways to do it. For example, if an existing top tier conference split into several (e.g. say that NeurIPS decides to hold 3 conferences a year, NeurIPS-America, NeurIPS-Europe and NeurIPS-Asia, or whatever) in practice they would probably be top tier from the get go.
Many research organizations use formal conference/journal rankings. These are usually calculated by following h-index values for papers published there. A new conference would start as unclassified for a few years and could not be used when you need to hit some formal criteria in academia.