The reason to bring Pally/Druid/Shaman for healing (depending on which faction you were) over 15 priests basically boiled down to the fact that you couldn't feasibly gear 15 priests. Sure you got innervates, auras and blessings, but from a numbers standpoint it was basically ideal to roll with 10 priests and a mix of a couple druids and paladins if you could recruit geared Priests.
Similar reasoning to not bringing 10 rogues, etc. The difference between a well geared Rogue and a well gear, say, Ret Paladin was literally like 100% DPS if you were playing well. And there was no such thing as a well geared Ret, because there were no set bonuses.
I will admit that the classic servers intrigue me, because I had more fun playing Vanilla than anything else. But I think people remember Vanilla so fondly largely because
1) They had no reference point to compare it to, since most people had never played an MMO, or had just played EQ or some garbage like that.
2) It was a revolutionary game for 2004-5 that came out at the perfect time when a majority of the country was just getting high speed internet at home making large-scale online gaming possible. Raiding with 39 other people was just so cool and new back then!
3) There is a trade-off between immersion and convenience in MMOs. Vanilla was all immersion (long runs to dungeons, slow progress, giant world that felt alive and like nothing you had ever seen in 2004). Current game is all about convenience, because attention spans (and the lack of awe-factor at big raid groups and big open worlds 15 years after WoW was developed) are now what they used to be. The immersion was what made vanilla special, but doesn't really fit in 2018 when all of that stuff is commonplace in gaming.
If they release Vanilla with no changes, I literally can't imagine more than maybe 20-50k people sticking with it for more than a month or two despite a very large surge of players initially. Certainly the bulk of players will never make it to 60. And those who do make it to 60 will likely:
1) Be VERY disproportionately Priest/Warrior/Rogue/Mage, with very few Pally/Druid/Shamans around
2) Be so much better at mechanics that spamming your 1 to 3 button rotation while doing 1 mechanic per fight will mean the only walls to progression will be the incredibly slow gearing of 40 people @ 2-3 items per boss. Mechanically every boss in MC/BWL/1st half of AQ40 is a first-boss of current tiers (or easier) level of mechanics. People will get bored fast
I think in order to make the game have any sort of lasting appeal, they should leave the class functionality/rotations/etc the same, but tune numbers so that at very least hybrid specs are like within 25% of main specs. Resto druid mains are not going to put up with being literally useless in the open world for very long in 2018 or whenver the game comes out, unless they love Vanilla to an absurd amount. There are players like that out there, but not many.
I don't think many changes need to made to the overall game, I like the idea of a real vanilla experience. But making it so that Druids/Pallys/Shammys are at least somewhat functional at non-healer roles seems like a much needed tweak on the numbers side only.