MMO stands for Massively Multiplayer Online (to state the obvious).
It’s does not stand for “Must be in pre-made group and Discord at all times.”
Playing “Solo” could refer to simply doing WQ’s or Queueing solo via a matchmaking system, and everything in between.
Personally, I throughly enjoyed playing both Cata and MOP “Solo” yet still did plenty of group content. It was just mostly via LFD/LFR and the odd PUG. LFD had nice welfare gear for Alts and LFR still had Tier sets. I didn’t care about my Ilvl or how I compared to Top Raiders, I just had fun.
Modern WOW has stripped away that fun over the years as it slowly eroded the rewards from queueable content, including PVP. Naturally, I went back to organised raiding, probably as Blizzard intended, but as is often the case, people quit, guilds collapse and I’m fed up of paying for server/faction transfers to join new guilds, and without the same fallback to Solo as I enjoyed in past expansions I simply quit.
As have many, many others.