The Vref+20 has been around since the introduction of the B707 well over 50+ years ago. Back then engines had slow spool-up rates, flight deck instrumentation was 'steam driven' analogue dial gauges and the pitot/static system was rudimentary to say the least. Today we have engines that have good spool-up rates and loads of excess thrust, instrumentation is digital coupled with a much improved pitot/static system, means that there is no need to be carrying an extra 5 knots (Vref+15 is ample).
As tttoon says: why place your Vfly close to flap placard speeds and risk a flap load relief?
Moving ahead with the times!!