While many agree with you, it should be noted that for computational reasons, astronomical year numbering and the "ISO 8601" standard designate years so that AD 1=year 1, 1 BC=year 0, 2 BC=year −1, etc. In this instance, a new decade would indeed start with a year ending in zero. It is this computational logic that also defined the last year of the last millennium as 1999 (and hence the new millennium was celebrated worldwide on Jan 1st 2000). I think most people can more easily identify with this logic and celebrate Jan 1 2020 as the start of a new decade. But if you want to be a stickler about it, I think your point is valid.