I don't have an optical drive on my PC, and would like to use the expansion as well.
It is not IMPOSSIBLE that this will happen. It's true that the guys who wrote (and thought of as their baby) the original game were very unhappy about their owners allowing a third-party to do what Blizzard North had claimed, in their original pitch, was part and parcel of the game design... expansions. They wanted to be the only ones to do this... but after Diablo, instead of creating the expansions they'd promised their publisher, they decided to create a whole new game (Diablo II). Which, itself, only ever got ONE expansion, though a good one IMHO.
So, the guys who made Diablo were unhappy from the start that someone else got to play in their sandbox. The odd thing is, "Hellfire" is actually an improvement on the base game, and a number of the things that they implemented for Hellfire showed up in Diablo II as well. In fact, they'd created THREE new characters, but were told that they could only use one of them (since two of them were characters that Blizzard either had thought of on their own already, independently, or actively stole from the Hellfire team... accounts differ on which actually happened). But yes, Hellfire had a "Barbarian" character, and they had what was basically an "Amazon" character (a "rogue with magic").
So, we get a new version (separately installed, not a "DLC" on contemporary terms, but requiring the original game CD to play) of Diablo, with several new features (running was added, just for one example), some new loot items, one new character (a hack can give you the other two which were removed, but they (as of now) use the graphics for existint Diablo characters). And it adds several new levels, which are outside of the main "dungeon crawl" and are accessed from the areas around Tristram. It doesn't actualy interfere with the main original Diablo gameplay in any real way, just "tweaks" it a bit in ways that consumers wanted.
So, it's hard for me to understand why David Brevik and his team at Blizzard North were SOOOOO angry about the game. He likes to bring up the "teddy bear quest" but that's another thing that can be restored by "tweak" to the game... and it's actually NICE. A girl in town asks you to rescue someone who sure sounds like her brother. It turns out that one of the monsters stole the girl's teddy bear, and after you slay the monster and his clan, you recover it along with some loot. It was actually a "feel good" moment to return the teddy bear to the little girl... and it didn't clash with the overall feel of the game at all. It simply wasn't as "dark and brooding" as everything else in the Diablo-verse apparently has to be. (Remember, these are the guys who turned your characters from Diablo 1 into enemies you have to kill in Diablo 2... and who turn your armorer in Diablo 1 into a high-level zombie "boss" in Diablo 2. They tend to be VERY, VERY DARK.
Personally, I was fine with Hellfire. My main issue with it was the "Sierra installer" routine, and the fact that it had to be installed to a "Sierra" directory... which was sort of stupid, IMHO.
But as pointed out, above, Blizzard (as it existed then) no longer exists... and Blizzard North is long since gone. Everyone who "hated" Hellfire is gone. And the same "holding company" owns both the remnants of old Blizzard and the remnants of old Sierra... so there's no LEGAL impediment to selling it, and nobody with a vested interest to NOT sell it anymore. The parent company will sell it if they think they can make $$$$ from doing so, in other words.