Star Wars is Lucas' creation, and for him, every new "enhanced" version is some sort of update, not an alternate version. From his point of view, it's normal that he doesn't want to release the O-OT on DVD.
But don't misunderstand me, I'd love to have the O-OT on an official DVD too!
And you're claiming that Lucas considers Star Wars a commercial product and "he fully depended on the commercial momentum the OT still had". If it was the case, don't you think that he would already have released every movie in several versions (I'm talking about DVDs)? From a purely commercial point of view, it would be very fructuous to release the original versions of the movies (including TPM and AOTC), as well as the "final" versions, and maybe the OT SE. In this case, many fans would have bough 2 or 3 different versions of the same movies -> pretty commercial, isn't it?