I think it's less a case of Sig losing their way, and more a case of the market dictating their path (at least to some degree). People want compact, striker fired, low recoiling, light-weight polymer guns, that are competitively priced. Me? I just bought a P224 precisely because I like the design of the DA/SA system, and like the extra weight to offset the recoil of a .40 S&W (and hopefully .357 Sig). But that's not what most people want. And if the P239 is being discontinued, it's not because it's not a great gun (it surely is!). It's because it has a single stack magazine, is relatively heavy, and quite expensive compared to other products on the market.
I can't fault people for wanting a sub-compact, light-weight, high capacity (for the size), simple to use, and affordable polymer pistol in 9mm. So it's hard to fault Sig for following the market. However, I do agree with your assessment of the quality of the original platform.