No doubt he could've made LHW, but I think the stark reality was that there was no money to be made at that weight. Not necessarily because of the lack of opposition, but more because LHW was always seen as a "Cinderella division". I think Foster came at the end of Henry's time , and even if he had fought and beat him (which is not a given, Foster was an ATG LHW.), he probably would've still made more money fighting Ali, London, Bugner later on and even being a sparring partner to the top HW's .
He would never have become the face of BRUT by being LHW World champ.