Who would maintain such information repositories and what would the incentive be to take that on? (As they no longer could be supported by ad revenue.)
Resellers maintain such information. There are many websites where you can specify the weight, height and hundreds of other parameters for your product search.
Ad revenue is not the only kind of revenue. Traditionally, people would pay purveyors for goods or services they deemed valuable enough to acquire/use.
Who would maintain such information repositories and what would the incentive be to take that on? (As they no longer could be supported by ad revenue.)