My five cents for a guess: they've used the email as an index somewhere, which massive amount of data now depends on and they keep pushing forward the need of migrating away from that because it'll be painful and take long time.
That's a good guess .. but I bet they are also paying price for this in support costs .. I'd be curious what percentage of their support tickets are related to this (also the require you to cancel subscription, create a new account, create a new subscription if you want to change a pro account)
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