This is true, but only for the set of messages that produce value that can be captured by the advertiser. This set is a small subset of messages that produce value. For example, a message about the benefits of excercise/socializing/climate action would produce a lot of value, but not in a form that any single advertiser can capture. So a lot of high value messages don't get produced in the current system, and might have a better chance in a more "natural" attention economy.
Advertising also increases the value of a product, so the value of things whose value can be captured by advertisers will be inflated when compared to their value in an environment without advertising.