No, that's not true. There's a reason why Delta has been the main variant of interest for a year and now we're looking at Omicron, several Greek letters later. All of the intervening letters have been used (except Nu and Xi) because new variants have arisen and looked like they might do something, but haven't really gone anywhere.
Take Epsilon for example, which was a major variant of concern in California, accounting for around a quarter to a third of all cases in the state between September 2020 and 2021. It seemed more infectious, but it has actually now been outcompeted by Alpha, the first variant. Iota did the exact same thing but on the US East Coast, becoming highly prevalent pretty much everywhere in the USA before falling back behind Alpha and Delta.
The UK had a wibble about Gamma a while back, and also Eta, but both ultimately went nowhere. Delta and Delta+ are still our major variants of concern, and they've beaten everything.