Hybrids are entitled to do as much or more dps as you. In the days of Vanilla, the argument from your side was that pure dps brought little to no buffs, which means raids brought you only for your dps. This was more or less true, but to be fair the only hybrid classes actually used for dps back then were warriors, shaman, and maybe a priest. Also at that time hunters and warlocks, both pure classes, were complete shit. Hunters were there for tranq shot, and locks were there to give curse of the elements to the mages, and a well geared warrior was better at dps than a rogue. So while the "hybrid tax" was there on paper, it didn't really exist in reality as class balance was all kinds of fucked up.
Come TBC, we were a lot closer to the "perfect" balance where pure dps performed very well with hybrids being viable but not the best. We saw hunters, warlocks, and rogues all do extremely high dps in this expansion(mages were sadly shit compared to warlocks in this expansion), and many hybrid classes actually had viable dps builds that brought important buffs(thank you ret pallies for giving my hunter infinite mana back then). Hybrids weren't the best dps back then, but they sure as hell could compete.
Then Wrath came along and suddenly hybrids were doing as much if not more dps than pure classes. This is when I remember the debate about pure vs. hybrid reaching its peak, and I was very passionate about it as I played a DK in this expansion. Hybrids still brought more utility than pure classes at this exact point in time, which I'm sure is why a lot of pure class players were whining back then. While I did sympathize with my pure class friends, I always maintained that I was entitled to perform as well as them simply because I could only perform one role at a time. If I tried to go into frost presence while in dps gear, I would just get destroyed in a second by any boss in the game. If a druid in healing gear tried to go into bear form to tank after the MT died, they would also get destroyed just as easily. Also many guilds didn't have their hybrid players perform more than one role. I was mostly a DPSer for my guild, but I was called on to MT a lot of fights such as 3 drake sartharion, General Vezax hard mode, and I had to OT Iron Countil, Thorim, Alone in the dark, and a bunch of other fights. So I had to dedicate a lot of resources to not only learning to tank and dps, but I had to have two very good sets of gear. This is essentially like having two characters as the really good tank gear always went to our MT first, then me. I would argue that this is essentially the same as playing your main pure class then going on an alt raid with your hybrid alt so you could bring it in on a fight if the guild needs it as my gear for my second spec was never ever as good as our MT's, and in a sense your alt would actually get the best gear for tanking if it was a tanking alt - possibly making an alt's main spec gear better than my main's second spec gear.
What I was trying to say with the above paragraph is that if I can only do one thing at a time, and I can only do my second role well if I am supported extremely well by my guild, why shouldn't I be able to be as good as you? Plenty of people were in guilds not nearly as good as mine, and they surely wouldn't have great secondary spec gear, so why should they be punished for being able to do a second job poorly?
Then the patch before cataclysm came out and basically every buff is given by several classes, making the utility that each class brought not all that relevant. Which in turn makes the "hybrid tax" extremely obsolete.