PS4 Pro is an addition to the PS4, not a replacement. You shouldn't expect sales to double, but you should expect sales to improve.
For example, the PS4 sells 100,000 a week. With the Pro, there are some possibilities. One, Pro eats PS4 sales without generating more interest.
So now you are still selling 100,000 PS4's, but some amount is Pro, the other isn't. This isn't good as that could either mean overall demand is falling but Pro is able to keep sales afloat or demand for the Pro is negligible.
Another possibility is sales improve. Demand for the PS4 stay about the same and there is an opportunity for hardcore buyers and enthusiasts to buy the Pro.
Now there are 100,000 PS4's and some amount of Pro, or just +100,000. Thus is good as now overall hardware sales are up and more chances to make profit.
Then there is the least likely option. Somehow the Pro opens up this massive untapped market that never bought a PS4 at $399 and now both the PS4 and Pro are selling about the same.
100,000 PS4 a week as well as 100,000 Pro a week.
Slim sales will improve in the holidays as, like in every generation, people hold out until a device is cheap enough. Keep a product at the same price forever and sales will crater as the market for $400 machines will be tapped. Slim will be $249 this Black Friday and sales will massively improve. Now with the PS4 Pro, out of the +40 million current PS4 owners, that reopens a new way for them to rebuy into the ecosystem that wouldn't exist before. This is a smart way to circumvent the traditional slowdown of sales past the peak year.