You're arguing that competition is sufficient for quality, not that it's necessary. The first vendor might choose to provide a high quality product regardless, which could, in itself, provide a barrier to entry preventing competition.

The real win of competition is that it allows a market to be commoditized, at which point it actually operates like we all learned in our Econ 101 class.

I wouldn't say that it is required, simply that it is more likely to improve quality.

I think droz mean "You don't need competition to do a good job" rather than "It's better to have competition when you want buy something of quality".

But in either case, sometimes, competition lowers at the same time the price and the quality. So competition doesn't necessarily increase quality.