I mean, if they are fleeing, they are fleeing with their money. Capital is essential for an economy and if capital leaves the country, it means that you have less growth, less investment and less prosperity in general. You can’t even tax that capital once it has left the country.
Plus, many of those low-millionaires are probably some of the most competent and knowledgeable people (not the hundreds-million industry captain with ties to the government, but the plant manager or lead researcher, lead developer etc. i.e. those who’ve made a small fortune through their ability). Getting rid of lead people is not exactly beneficial for an economy.
And sure, making everyone poor will reduce apparent wealth inequality, you’re right.