I don't think you can say anyone has "assumed" the W/T will dissipate quicker, they have done years of measuring at Heathrow to prove that it does disippate quicker in stronger winds.
This isn't something that's been made up, there is a lot of hard work and data research behind it. The article mentions four years of work and data gathering from 150 000 flights.
I agree though, it will be interesting to see if it has an effect on the amount of WT encounters, and if that is an increase or decrease (as in some winds spacing will now be increased from before, as Gonzo mentioned above).