McDonnell tells us that above some £70,000 a year people should be defined as rich - or more accurately, as high income. There are problems with this, that sum in Central London isn't a fortune while in Middlesbrough it will finance a very much grander lifestyle. But then that problem is always going to happen in a national tax system. But McDonnell's point was that we should be expecting those more fortunate to be contributing more as a percentage of their income to the public coffers. And we've not got a problem with the idea that the top 5% should be identified as those who should be contributing more.
Adam Smith did after all talk about more than in proportion to income. Our own desired taxation system is rather different of course, with a substantial tax free allowance and then a flat rate but that still allows progressivity. But that essential underlying logic McDonnell is using seems fine to us.
But there is a problem here. As the Diamond and Saez paper points out the peak of the Laffer Curve in a tax system with allowances - and yes, being able to leave the country and thus the tax system is an allowance - is 54%. And this is taxes upon income, not the income tax alone. So, employers' NI must be included. Meaning that we are above the peak of the curve already. There is therefore no room for another higher rate on those over £70,000.
Yet that moral intuition that the richer should pay more still exists and is still correct. The only possible answer therefore is tax cuts for those on less than £70k. Which is entirely fine with us we shouldn't need to have to point out.