Classic is hard? I don't think so, go look at clear times for various raids in Classic WoW, Classic BC, Classic WotLK, it's nothing compared to Vanilla, BC, or WotLK the first time around.
Additionally, let's look at what you said:
Emphasis added. Those raids won't work "just fine" as they are scaled up (without additional tuning passes) to the current content in terms of character level. As illustrated by the Molten Core LFR for the 10th Anniversary Event. There was ZERO tuning to that Vanilla WoW raid, the devs said as much. The only thing that changed was scaling the mobs up to the level cap (which at that time was level 100).
The wipe rate was incredibly high because even though the mechanics were simple (relative to the mechanics we have now in raids), which further impacted participation rates. Also probably why we haven't seen Molten Core LFR come back at all.
It's incredibly naïve to think that you could just take Throne of Thunder scale it up to level cap of DF with ZERO tuning passes. Heck, just look at the "older" dungeons that are now in rotation for M+. How many tuning passes were done on the older dungeons?
Honestly, if players crave the older raids, Blizzard already has you covered: "Just go play on the Classic servers."