I think it’s a possibility, but also the big thing HC has to keep it going is that people always have to “go agane” so it doesn’t need to release more expansions to keep people’s interest.
And TBC just makes the main aspect of what most people enjoy and can always participate in, which is leveling, a longer grind. I guess that does make it more challenging and give more for people to play, but if you ask me I don’t think I’d want to start fresh and go through all the Classic content again just to get to go through TBC content. And TBC updates would make classic even easier and sort of water down the experience, plus while it’s still very possible to die in future expansions if I’m being honest the game only gets easier and more friendly towards leveling as each expansion releases. TBC is probably the only expansion I’d ever be worry about dying in and after that I think any deaths could really only be attributed to unforced errors.
Personally for HC I think finding new ways to make Classic fun would be a better use of time such as new challenges like the addon had, changes to scaling, even removal of some abilities (such as taunt), etc.