In general, I agree, though I think I'd also include some consideration of size or barriers to entry. As an example, photomultiplier tubes are used to detect very small amounts of light, down to a few photons. They are primarily produced by Hamamatsu, which has about 90% market share for PMTs. And they are ridiculously reliable, used in detectors that last for decades. Even without competition in PMTs, they make good products. It's a very small market, and doesn't have room for much competition, so I don't think that market share alone can be the sole determinant.
That said, if the quality were to drop, if they were to engage in anticompetitive behavior, if they were to provide tools to subvert an election, if they were to repeatedly buy out competitors, or any combination of the above, I would drop them in a heartbeat.