I believe GP means because if transfer is allowed, it'll happen at the expense of 'new' sales, which hurts developers/studios/publishers as much as it hurts Valve, so they adapt model to mitigate and sell subscriptions instead of perpetual licences.
They have two clear options:
a) Switch to a true subscription model, but have to build a way to transfer items sold before the switch in payment model, as those have been ruled to be purchases. The gaming community would likely get annoyed with them.
b) Build a way to transfer items. They get to take a cut of this, and can also pass a cut on to the studios, whilst pointing to this ruling as a reason why it has to happen. The gaming community would celebrate it, even if the transfer is ridiculously complex or difficult.
The second option is the cheapest, and can only increase Valve's revenue, whilst they can also placate their partners.