nerdy thought

Since English doesn't have a reliable way of writing out the "th" sound in "those," and usually if an unfamiliar word defaults to the unvoiced sound (like in "thunder") does this affect the production of novel words such that no new words can ever have a voiced "th?"  If we assume that this is the case, then we should we really consider the voiced "th" a phoneme in English? If it is a phoneme why is it that English speakers can't call my father "Thimitris," or pronounce my last name as Kakathelis? It would totally boggle the mind of English speakers I'm sure. 

If a stable trascription was introduced, like "dh" as has been suggested in the past by people trying to do spelling reform, would this eventually allieviate this issue?

I think about this kind of stuff way too much.