lol, no, I'm saying you would have to get done with 1990 for the decade to be over. This is so easy to grasp.
Again, the calendar starts on Year 1, not Year 0. A full decade is 10 years. If we look at Year 1-10, you had to get through the 10th year for the decade to be over with. If it was New Years eve on Year 9, going into Year 10, it wouldn't be a full decade, as you only went through 9 full years. After you finish Year 10, a full decade would cycle.
Think of it like a tv show that has a season with 10 episodes. If you watch the first 9 episodes, and are going into the 10th episodes, but haven't watched ep10 yet, you haven't watched the full season, as you have to get through the 10th episode to finish it.
2020 is the finale episode of the season/decade, and we haven't gotten through it yet. We will once we get to 2021, as 2020 will be "the finale". Once 2020 is done, the decade will be over. If we stared on Year 0, then yes, the decade would be done, but that's not how the calendar works. The decade isn't done yet. Again, 2020 is basically the finale episode that we haven't watched yet. We have to get through the year, the same way you have to watch the 10th episode to finish the season.