I tried to defend a similar argument in a private forum today and basically got my ass handed to me. In practice, not only would modern P-curve implementations not be "significantly weaker" than Curve25519 (we've had good complete addition formulas for them for a long time, along with widespread hardware support), but Curve25519 causes as many (probably more) problems than it solves --- cofactor problems being more common in modern practice than point validation mistakes.

In TLS, Curve25519 vs. the P-curves are a total non-issue, because TLS isn't generally deployed anymore in ways that even admit point validation vulnerabilities (even if implementations still had them). That bit, I already knew, but I'd assumed ad-hoc non-TLS implementations, by random people who don't know what point validation is, might tip the scales. Turns out guess not.

Again, by way of bona fides: I woke up this morning in your camp, regarding Curve25519. But that won't be the camp I go to bed in.