That wouldn't be an entire bad idea, given the absolute prevalence of rain in this metagame. The issue is, however, that Mollux was intended to be a water and rain counter in the first place, yet it seems like it's much, much more frequently used in favor of rain rather than against it. Making a rain counter would potentially end up as another Mollux--used for rain rather than against it. Generally speaking, our next CAP would either be an overall weather-slayer (with a bit of bias put against rain) or just something different entirely.
I would love to see how well Mollux would function in a metagame without Politoed, however. Experimenting with the CAP metagame that way (which would be entirely fine given how CAP is essentially Smogon's metagame to be altered however we see fit) would not only give ourselves a temporary fix to rain, but also let us see if Mollux still functions well in its default use of being a Water-type counter, no Drizzles attached.