Not to be overly contradictory, but Smart Content is a Marketing Pro feature because it IS available on HubSpot Landing Pages (LPs) which Shopify users of HubSpot Pro can (and should) certainly use to market their products / services. Honestly, can’t imagine anyone paying for HubSpot Marketing Pro and NOT using LPs. Wow … who does that?
To your point that it’s not really reasonable to expect HubSpot to totally implement smart content outside of the HubSpot ecosystem -- completely agree. Can’t understand why HubSpot would ever even consider expending dev resources doing this since it flys in the face of the all-in-one methodology upon which HubSpot was built.
Just my opinion as a non-HubSpot employee.