I imagine that with a perfect (or even a "good-enough") upbringing a person would neither be too attached nor too detached, but be able to maintain healthy harmonious and appropriately boundaried attachments with a variety of different people who have different roles in their life.
I imagine when people have overly intense attachments with their therapists it is often (but not always, I suppose) because they were not able to develop a "good-enough" balance of attachment in their primary attachment relationships with their childhood caregivers.

Is it "ideal" to have an overly intense attachment to one's therapist? Not really. But when people have inadequate attachments in childhood an intense attachment can absolutely be triggered by the therapy relationship.

I see an intense attachment with the therapist as a necessary part of the journey for those folk, rather than the destination.

But I have the opposite problem, I am too avoidant and defended against attachment. It's equally not desirable, but likely much more tolerable for avoidant therapists, like my own.