OK let's imagine that investment returns will double my money between now and retirement. I could be paid £1,000 (less tax) or I could put it in my pension.
If I draw it now, and then put it in an ISA until retirement, I would get this:
1000 x 0.6 x 2 = 1200 at retirement
If I put it in a pension in your "20% relief" world I get this:
1000 x 0.8 x 2 x 0.8 = 1280 i.e. almost the same as drawing it out now, but losing the flexibility to dip into it earlier if i needed it
(This ignores the tax-free PCLS but since we are talking about high earners and the PCLS is limited many of them will have already hit their PCLS limit anyway).
Compare it now where I would get:
1000 x 2 x 0.8 = 1600 i.e. pension is almost a no-brainer. But the benefit to me drops from £400 to £80 at which point, as I said, the benefit of tax efficiency begins to be overtaken by the loss of flexibility