The problem is not so much that some rich people are now somewhat less rich. Boo hoo. Let's rephrase that another way though.
A lot of people that previously had the wealth and assets to invest in new businesses, grow existing businesses, create jobs and fund the development of new technology now don't. These are the primary ways wealth is actually used, and now there is less of it around to do those things. So a lot less of those things are going to happen now. If you either work for a company that pays you, or have customers that buy your stuff, this is a bad thing to happen. Companies will have less to pay you with, and customers will have less money to buy stuff with because the same applies to them too.