I'll agree with the opinion that it will be several years. Look at how long the P38/P1 was Walther's main offering - decades. There were refinements here and there, but they had what they considered to be an optimal gun that sold well enough and pleased enough buyers to keep producing them.
Secondly, as already mentioned - many of the other companies' latest guns are simply offering features the P99 already has. Glock improved the grip with grooves (although some people hate them), the M&P and the Ruger SR9 have various backstrap options, etc. I'm not saying they're playing catchup exactly... however, there are few things being done by anyone that Walther hasn't already done. The only real advance I can think of at the moment is Sig's new gun with replaceable frames.
As to caliber, my gut feeling is that offering 14 different sizes and calibers (ala Glock) is a distinctly American thing - and the market is saturated with choices if you wish to shoot an American caliber. Most of Europe is quite happy with 9mm. It's the main caliber for the US Military, NATO, etc. Of course, .40 is a new favorite of American LEOs, but the P99 has already covered that base.
Historically, Walthers has mainly been a 9mm company. You've had smaller things (ex: .32 and .380) thrown in there, but they've never made a big move towards something like 45 ACP.... and I don't see them embracing .357Sig or .45GAP merely on principle.
The PPS answered a need that a lot of people asked for - a singlestacked concealable gun. I suspect you'll see a .40 version, or perhaps even different trigger options (AS?) for the PPS before you'll see the P99 replaced with a different weapon.
Walther isn't Glock - they don't make a gun for every purpose.... fullsized for carry, longslide for target practice, automatic 9mm for wasting ammo, midget .45 for CC, etc etc. I think that Walther long ago decided their niche was combat handguns, with a small deviation to the CCW crowd here and there.
Just my option though.
thorn