If the numbers at https://dqydj.com/average-median-top-household-income-percen... are correct, the "top 1%" household income threshold in the US in 2020 is $53lk. $400k is closer to the "top 2%" level.
More importantly, these numbers like "400k" tend to not be inflation adjusted (e.g. the cutoffs for the net investment tax and the additional medicare tax are not, cutoffs for phaseouts of the child tax credit are not, etc). And income percentiles grow faster than inflation. Looking at that same data table, it looks like the percentile boundaries grew 6%+ year over year from 2019 to 2020. If that's not a fluke caused by the pandemic in some way, then in 10 years you expect something like 1.79x growth in the cutoff numbers, which would put $400k at about the "top 8%" level at that point.
Is it possible that your idea of what level is "firmly in the 1%" is just a few years out of date?