I guess my concern with one tax that covers everything, is that weathly people will just change what they do.
I don't think they are going to stop moving money around. In any case I don't thing "they might do such and such" is a good excuse to not do something. If they do something we'll so something too.
My suspicion would be wealthy people pay accountants lots of money to work out how to collect wealth with fewer transactions, and continue paying minimal tax compared to their wealth.
tax evasion is technically a crime but the tax is based on a percentage so it doesn't matter if you do ten transactions or one, if it's the amount of money.