It's actually not outside the realm of possibility that they might be able to announce 25m sold-through at Gamescom next week. Unlikely, but not impossible.
It's already behind Wii, and though it might start to close the gap in later years after the Wii fell off, PS4 likely won't live long enough to truly catch up.
It's still ahead of PS2, and will probably stay ahead until next spring. But it will fall behind, and there's pretty much no way it'll have the longevity of PS2. The PS4's final numbers will be somewhere in the range of half that of PS2.
As said above PS4 exceeds PS2 now, but is falling behind not blitzing ahead. Remember that PS2 launched only in Japan and had production problems for a while; PS4 launched much wider and had far more units available for sale. That provided an initial advantage which is being eroded away.
Yes, PS3 has almost no chance to hit 100m or top Wii. It might be possible if Sony aggressively cut the price and promoted it hard for customers less concerned with the cutting edge. But that's very unlikely due to manufacturing costs and other factors; it wouldn't make financial sense to take the hit just for meaningless bragging rights.
We'll probably find out a number eventually, though. They might announce when they pass 90m--they have a history of milestone PR. But even if not, it's likely they'll announce a final number whenever they discontinue production.
