I'm really wondering how many (or what percentage of) devices actually need "a special plug-in". Seems like it would be a very small - and shrinking - fraction, but I could be wrong.
If the user's device is (presumably) already set to use Mandarin (or any specific language), then I assume that would be the default input method, so why does that take significantly longer to use than numbers? I'm genuinely curious, never having experienced that use case myself as a native english speaker.
As for websites wanting to expand internationally and avoiding unfamiliar characters, that doesn't really make sense to me - if they want to expand internationally, I don't think the numeric domains are going to help much (they'd be just as cryptic to me as Mandarin characters), so why wouldn't they just register alternate domains in the target regions/languages as many already do?