From a pragmatic perspective, catch could perfectly retain the converts in their ranking system if there is a dedicated playerbase, while taiko and mania could simply deactivate pp on converts, because
1) the algorithm for conversion is suboptimal
2) the resulting maps are of awful quality
There are mode-specific ranking criterias for a specific purpose: establishing a minimum of quality standards of what goes and goes not to ranked. Converts in taiko (probably in mania too) break almost every single rule that we have established in the RC, yet they somehow award pp.
Speaking of pragmatism again, I mention specifically that taiko and mania can live without converts in the ranking system as of today, considering that there is more than enough content as well as a constant flow of new content to allow the modes to self-sustain. The most utilized argument in the past against banning converts was that it reduced significantly the amount of possibilities a player had to experience the game mode, due to the lack of mode-specific difficulties. Nowadays, not so much.