Sure they would be: properties that are valuable more than a handful of years after they are first published are rare enough that the chance of achieving that is a minor factor in motivation, but the existence of such properties under extended protection both reduces the potential for and reduces the market value of new works, reducing the motivation for new creators more than the slight chance of producing such a work increases it.

If expirations were shorter, say 20 or 30 years after the publication of a work, wouldn't creators be motivated to keep creating so as to not suddenly lose a stream of income?

Disney would have had far less material to lift.