I'd like to know, based on your opinions, what is the point a game goes from Multiplayer to Massively Multiplayer.
The "Massively" was added when games started crossing the 100,000 concurrent player boundary.
Knowledgeable programmers using off-the-shelf tools can support double-digit number of players easily. Multi-user games have been around since the 1970s. The Networking forum FAQ includes an example of such a server written in an afternoon.
Supporting a few thousand concurrent players is more difficult, but still generally not overly complex. With a little bit of work the biggest problems can be dealt with. The n-squared issue is the biggest killer -- players and clients within range of each other all need to notify each other and communication grows exponentially -- but at these levels it can usually be dealt with by some clever spatial partitioning.
Transitioning from thousands to tens of thousands can often be done by just adding another server instance. Amazon, Google, and other infrastructure providers can do this automatically. Spin up another service and all is well.
But when you start to cross over the bigger boundary, somewhere approaching 100,000 concurrent users, there is an incredible transformation that takes place. It is no longer a case of just "add another instance". The complexity of managing the infrastructure is a major task in itself. That's the transition that the business side considers "massively multiplayer".
Sadly for players, "MMO" and "persistent online world" are often synonymous. The term "MMO" is also used by players whenever a few hundred people are in the world. A slither.io server with 500 players is called "MMO" even though it is nothing of the sort.