How are these old posts invaluable? Do you think a paywall would have generated enough revenue to pay for serving them? If nobody is willing to pay for access, doesn’t that mean they are in fact not valuable at all?
As to who would pay for it, that's a tough one - my employer wasn't willing to. I suppose it acts as a commons
Deleting this is madness.
The calculus has changed a bit. They are losing sales from people using older versions of the software that they wouldn't have provided support for anyways. In Adobe's mindset, for example, Photoshop CS6 is the poison that is holding back subscription sales, not their competitors. So an easy way to frustrate users into subscribing to the latest version... delete the old forum posts!