At its height Pathifnder had around 250k sales of the core book perhaps more.
If they all went back to Pathfinder and each took a friend that is potentially half a million sales for Paizo. Since I doubt 5E has sold more than a million copies (otherwise Mearls would be claiming he has outsold 1E or BECMI) that would not be good for 5E if they lost that amount of players (Paizo would be ecstatic).
With WotC estimating 9 million players. I think it's safe to say that they've sold more than a million PHBs. They're probably nearing 2 million. The reason he's not claiming that is likely the absence of solid sales numbers of those early products.
So half-a-million players would only be 5.5% of the D&D audience.
But, of course not every Pathfinder player swapped to 5e. And that 250k Pathfinder moved took multiple years of continued growth. And, again, my number was a purposefully exaggeration. Quite a lot of people who played PF and now play 5e will not move to PF2. Likely most.
Which is the catch. Generally, the GM who picks the system they want to play. And 5e is a much easier system to run and manage than Pathfinder. And, likely, PF2. Very likely, a lot of PF alumni are not going to swap back. And <100% of PF1 players are not going to switch. Initial sales of PF2 are probably not going to be 250k.
1. PF2 beats Starfinder
2. PF2 beats 5E on Amazon slaes for a month or 2 perhaps the 1st quartly
3. Paizo release a good intro adventure and AP.
4. PF 1 and 2 combined go up 5-10% on the VTT's.
Starfinder's sales numbers on Amazon at its
height (i.e. right after release) are below what the 5e PHB is right now, 3 1/2 years after launch. And the sales for SF had a hefty spike followed by a steep drop (which is common for every RPG other than D&D). PF2 will likely only beat D&D on Amazon if D&D is having a bad month.
As for Percentages... D&D has been gaining an average of 4% per quarter. So 5-10% would be a
ridiculous spike. But even a 10% spike entirely at the expense of 5e would put D&D where it was in Q4 2016 and Pathfinder back where it was in Q1 2015. Even a 10% jump doesn't allow PF to reclaimed its percentage of the VTT market it had in 2014.
I imagine PF1 will drop from 12% of VTTs slowly to 1% over the next 18 months (if it keeps dropping at its current rate, it'd be at 4% by the time the game releases, but it's percentage will decline faster as people swap). While PF2 *would* go up to 10-15% if not for the continued growth of D&D that likely just causes D&D to appear like it's plateauing while PF has a 0-3% bump.