The less well off Higher Rate Taxpayers (they are not all millionaires) may not be able to pay so much into a pension fund, and therefore reduce its worth for all contributors!Yes! See post #93 from pendodave. After all many who gain the higher relief will revert to standard rate tax post retirement.
Thus they would in your scenario continue to invest in largely tax exempt funds and receive exemption from higher rate tax on those contributions whilst only paying standard rate tax on the taxable portion of benefits arising from those contributions.
As for the effect upon funds I am afraid that there is none arising from this possible change, unless, of course, you are suggesting that higher rate tax-payers will not or should not contribute to their pensions unless they enjoy the benefit of full tax-relief.