From experience I can say it's not just lack of money that can kill the thing. A therapist can decide the attachment isn't healthy or therapeutically acceptable or of the right "type". Or they might get spooked. The list goes on.

You can find anecdotal evidence on this forum of people benefitting from therapy in this way (though they all seem to be still in therapy), and anecdotal evidence of people seriously harmed.

To me the most relevant evidence, as it were, is the basic structure of therapy. To me it does not conjure up visions of healthy outcomes. It looks very rickety and very sketchy. One wrong move, bam, pile of rubble.

Also I think there is a danger in talking about attachment problems and therapy in the same conversation. They are basically unrelated. A therapist might explain attachment styles and so on, but that doesn't mean therapy itself is any sort of evidence-based or reliable remedy for this.