Honestly completely disregardiing the benefits of anathema-elysium catchup sooner for TBC:
The original blizzard timeline does feel too slow for private servers. Like the OP says, blizzard had gating mechanics(bugged impossible final boss) that lengthened the content in a sense. Even though on Anathema we are getting Naxx at an appropriate time compared to AQ on the blizzard timeline, we have killed cthun almost 3x the amount of any guild in the original timeline and have almost 3x the amount of cthun loot in our raid team going in to Naxx. Same thing happens with Nefarian, and Ragnaros. When you consider that these final bosses have significantly better loot then anything else in the raid tier, it has a huge effect. We have way more full bis people going in to the next raid tier. In retail vanilla, guilds didn't have CLOSE to the amount of gear. Everyone didn't get FULL BIS from a raid before the next raid hit. That's just not how it worked. A couple of your casters had staffs of shadowflame and tears, not every single damn one. This is a PART of why the game seems easier then people remember (on top of 1.12 yada yada)
More important then the gear/easier next raid tier angle, the wait for new content is TREMENDOUS compared to retail vanilla because we don't have that impossible final boss for a few months keeping us on the hook. Cthun was patched and made possible ~3 months into AQ suddenly guilds start killing cthun each week, and then bam 2 months later Naxx hits. The flow of new content was better paced with less full farm time.
For the reasons of:
no gated content like retail had
12 year old game, players are on average better, all boss mechanics easily googled
1.12 classes from the start
I think the original 2 year+ timeline for vanilla is a mistake. You can easily fit vanilla more comfortably into 1-1.5 years. Additionally I think the best way to preserve this game for the years to come is a seasonal system. Have a fresh vanilla server at the same time each year, while the current vanilla season moves on to TBC. People can play every season - or every couple of seasons hop back in. I can't think of a better end-game in regards to preservation of vanilla wow.