Yes it is. The customer paid for their past use of the service with their data. Wanting to take that data away after having used the service is exactly like wanting a refund for a product you consumed.

Regardless, GDPR requires that you do this. Data's value is massively inflated in the minds of "technologists" and it is time that price came down to reflect the realities that the rest of the world wants.

Sincerely hoping that this marks the end of the data gold rush

they aren't getting the service indefinitely, why should you get the data indefinitely?

When you finish working at the end of the day, do you return your salary? When you leave college do you get your tuition back?

No it isn't.