As a warrior tank you dont replace anything before Kara, maybe some trinkets/rings/your neck.
So its most likley a big difference between classes, think rogues didnt replace most things until they were ether doing heroic dungeons or raids (and Kara you can do before going into a single heroic dungeon)
Only fools prefer TBC over vanilla.
Only fools prefer vanilla over TBC.
I guess Iām a double fool then. I prefer both of them. Over WotLK.
And thatĀ“s why when it was actually still current (and challenging) content everybody and their mother, including high end raiders, complained about Years worth of effort being nullified in under 2 hours? 
That was the whole reason they overworked their approach to item scaling in Wrath so that many if not most of us were still wearing T6 when we dinged 80, or our t10.5 until 85 in cataā¦
IIRC Warriors were the only ones that actively kept their Tier 3, literally almost everyone else replaced it with questing greens because they had almost twice the stats.
This is entirely correct, in modern expansions such as TWW if you have 515+ DF gear you will be wearing that until 69 or 70 when higher itemlevel gear actually starts to drop, as opposed to replacing it with greens that have twice the stats 10 minutes after entering the new zone. 
Unless ofc they have done significant changes to the itemlevels and corresponding statsā¦ I havenāt touched TBC classic and donĀ“t plan to, as I have good memories that I would like to preserve and not have 
on by todayās community
You are exaggerating quiet a lot, casters, who are most impacted by this, still only start replacing enchanted T3 with blues around level 65, specifically socketed quest and boss rewards from Mana tombs and Auchenai crypts. Then it speeds up with elite quest rewards and drops from lvl 68+ normal dungeons, steam vaults, shatered halls, bot and mech etc.
The āgreens you get in first 10 minā will indeed replace all old world levelling gear, as in stuff you go into dark portal if you hit it the moment you ding 58, yes.
I think that the most important thing is to find a guild. Solo wow experience is miserable.