It is literally exactly a monopoly, an unprofitable monopoly, but a monopoly.
Tesla has never had a monopoly on large premium cars. This is a mistake many people do, assuming that the BEV market is a separate market from the ICE market. However, it's certainly true that BEVs have significant competitive advantages over ICE vehicles, which is why it may seem like a separate market, and it's also why every BEV that is brought to market at a compelling price will quickly be sold out.
There is electric competition coming, but there's no meaningful volume planned:
2018, Jaguar i-Pace, ~20k/year, Model Y competitor
2019, Audi e-tron, ~30k/year, Model X competitor
2019, Mercedes EQC, ~30k/year (?), Model Y competitor
2020, Audi e-tron Sportback, ~30k/year, Model S competitor
2020, (Volvo) Polestar 2, ~20k/year, Model 3 competitor
2020, BMW iX3, ~50k/year (?), Model Y competitor
2020, Porsche Taycan, ~20k/year, Model S competitor
Currently, BMW + Mercedes + Audi+ Lexus + Porsche + Infiniti + Jaguar + Volvo ++ sells roughly 8.5 million cars per year.
Even counting Tesla and all the competitors coming until 2020, adding up to ~1 million cars with 83% being Tesla, the BEVs will only have reached a ~12% market share, and there would still be plenty of room to grow.