HBO is gaining a lot of traction outside of the US, especially in Scandinavia with HBO Nordic (HBO GO equivalent without a cable subscription). So they have the platform to do it, which means if the cable market starts plummeting they can pivot and start offering it as a seperate package rather than an add-on for cable.
I have hope that they will do this, but looking at the history it's far from certain.
My guess is this is exactly what they're planning to do, but that a revenue tipping point hasn't been reached in the US yet: cable subscriptions still provide too much money for them to be willing to flip the switch. Experimenting in Scandinavia lets them lay the groundwork without prematurely giving up the huge US cable revenue stream.
I have hope that they will do this, but looking at the history it's far from certain.