Because, getting back to the original point, stating "Mental health treatment has come an incredibly long way, just in the last couple of decades" without offering any substantive support for this viewpoint (regardless of whether one's anecdotes are "real world" or not) is, in and of itself, a valid criticism. The onus is on the person making the claim to provide a rationale to back it up.

What makes it not valid? It would actually be great to have an answer to that instead of the parent's complaints