I'm not sure if I'm missing something here, but many translation solutions support ResX files out of the box. For example, I use memoQ, which has built-in support for ResX files. Additionally, open-source solutions like OmegaT can handle ResX files as well.
Yes, resx support is finally becoming available in more solutions, but it's still not always a first class citizen. But perhaps that's the long-term solution, wait for resx to take over. Although it still doesn't support plural forms (then again my current setup doesn't either...).
gettext/PO is just as deficient at modern plural forms support as ResX, out of the box. The only format that I'm aware of that is specifically built to include it as first-class is Mozilla's Fluent. The same workarounds generally apply to ResX as to PO: create a number of separate strings and do a bit of math up front to choose the right one, or use a formatter on top of PO/ResX that supports something like ICU MessageFormat.
