I completely agree with you about every open wheel driver having dreams of F1 (I certainly did).
At this time in Herta"s and Rossi's careers though, I expect it's also about maximizing the income potential of a limited number of competitive years, in addition to the dreams of youth. Perhaps less so for Herta, as he is younger. Maximizing the career income means victories and championships, which require a competitive car.
Personal sponsorships can significantly affect the income however, and one would think F1 participation should provide more opportunities than IndyCar