Vanilla had players starting throughout its entire lifespan. (I started June 2006 - Naxx was opened that day. Everything else was available. I had no less fun progressing through.)
Only some 200,000 players were logged in on Day 1. Only 1.5 million were playing by 1st Q 2005 (Dire Maul was released). Only 3.5 million were playing by 2nd Q 2005 (Honor system, BGs). Only 4 million were playing by 3rd Q 2005 (BWL, ZG). That means half the people who were playing by the end of vanilla weren’t playing until several of the “stages” were already part of the game.
Phases aren’t necessary to enjoy all of what WOW Classic has to offer, because there’s no insane scaling like Retail that negates prior content. (Seriously … Retail has a hotfix/patch to up the iLvL of World Quest rewards leading up to a raid tier opening so that more people will meet the minimum to queue for it. Why go back to the prior raid at all? citation
Classic has the prior tier to earn gear from. Sure, an alt can get carried by friends, but the gear sources are still going to be the “older” content.
For many of us, the experience of leveling as each class is great fun. What end-game content is available doesn’t change that. Doing each dungeon from a different role, from a different class, gives a fresh perspective. Working on each profession, collecting recipes, farming for random world drops or rep to get a certain mount on a different race.
No, I’m not saying YOU have to enjoy all that. However, just because you wouldn’t, just because you’re constantly champing at the bit for new, shiny raids and eSport highs does not mean WOW Classic is, or ever should be, modeled after Retail in that way.