For the customer with an Internet "mission critical app", being tied
to a Tier 2 has it's own set of problems, which might actually be
worse than being tied to a Tier 1.
The key word is "might". In fact, I would posit that a Tier 2 with multiply
redundant transit to all of the Tier 1s could theoretically have better
connectivity than an actual Tier 1. The Tier 2 transit provides flexibility
that the transit-free Tier 1s do not have. Just my opinion.