> "Monopoly" used to mean a single company controlling a market. Now folks are now hollering "monopoly" about five different companies competing and collectively capturing 90% of a market?

Five companies controlling 90% of the market is more than enough to form a cartel or spontaneously self-organise in ways that have the same effect even without explicitly coordinating. (I once read an economics paper that I wish I could find now that said this would happen when the top 4 companies in an industry controlled more than 60% of the market)

What are the primary problems with monopolies? Do you think those exact same problems could be seen by a small group of of companies who act in a monopolistic manner (much like a cartel)?

I think you are stuck on the definition of the word monopoly rather than the reason behind why we ban/rule against them.