Well, having been through the armorer class for the SW99/P99 a few times when S&W was doing it, and having spoken with repair techs for both the SW99/990L's and the Walther America business at S&W, and having the later produced comprehensive armorer manual from Walther in Germany for the P99 AS, C & variants (P990, P99DAO, P99QA), I had the opportunity to learn a little about the 99 series of that time period.
S&W engineers shared some thoughts with Walther about the 99 series, which included helping them identify and resolve the early premature slide lock issue in the .40's (both the P99 and the SW99), which was addressed with a magazine body & follower revision. It was S&W's use of high-speed imaging which helped identify the way the top rounds were being displaced too far leftward under the .40's heavier recoil, jostling the rounds inside the magazine body, below and behind the slide stop tab window. That info was forwarded to Walther (according to what we were told by someone from the factory at one point). New MecGar magazine bodies and followers were forthcoming from Walther. Walther used different shades of blue on their .40 followers, and S&W used different shades of orange on their .40 followers.
Removal of the hooked portion of the frame above the web of the shooter's hand came about after S&W requested it in the frames made for them by Walther. The first .45 P99/990L models came about when S&W repeatedly requested the larger frame to accommodate the .45ACP for the American market. There was even a version made with a magazine safety, in case any LE contracts required it. We were told that variation would only be made by Walther for locked in contracts. Walther's own decision to introduce a .45 model in their 99 variant series came about some years later.
S&W decided to adopt the 'optional' extractor spring for their SW99's (and the 990L's), which became standard in the .40's, and the short-lived .45, and then the 9's. It was heavier than the standard spring.
I had to correct a failure-to-extract issue in a P99AS belonging to one of our folks, and then a SW999 (full-size) belonging to another of our folks. In both cases it was resolved by removing the original '9mm' extractor spring and replacing it with the heavier spring (considered optional by Walther, at that point, we were told). FWIW, the heavier spring was the same spring used in the magazine catch assembly. I called the factory to discuss using the optional spring in the P99, to make sure it was something they approved in both the SW99 and P99 guns at that time, and was told it was good. Once done, both the P99 and SW99 again extracted normally with the duty ammunition we used. That was about the time I noticed that the S&W99/990L parts list now showed the heavier extractor spring to be the standard production spring across all their models and calibers.
There were other revisions made over time by both S&W and Walther. Some were obvious, and some were subtle and required knowing what to look for, and a couple required being able to disassemble the frames as an armorer.
Personally, I thought that both model lines were fine examples of the newer generation of plastic-framed pistols, and admired how Walther broke the ground on introducing the use of replaceable backstrap inserts. The only reason I ordered a couple of SW99's for my own use was because I was told by S&W that they could offer more of a discount on the SW99's, because the P99's imported from Walther for the Walther America business had more cost (to them) involved, since Walther made the slides and barrels. Since S&W forged, through-hardened and produced their own slides & barrels, and had the Melonite nitrocarburainzing done in the US, they could offer a better discount on the assembled, finished models. I was budget-minded, so I went with the SW99's.
I have zero experience with any of the other licensed 99 variant models offered by a couple other companies, since I never had an interest in buying them, and never attended any armorer classes for them (or know if such classes have ever been crfeated, since those models weren't exactly being promoted to LE/Gov users).
I was sad to hear of the discontinuance of the 99AS (and the PPQ), but I can understand Walther's desire to try and stay current with the market trends.
On the flip side, I'm hopeful that Walther's shift to offering all-metal striker-fired pistols ... and S&W with their all-metal M&P ... may bring some well-deserved attention back to the use of metal frames for both working guns and competition guns.
Just my thoughts.