Consider the roots of English. It comes from Old German which goes back to Latin.
In Latin the "th" is theta. Theta made it into Old German but after English split off, "th" was dropped in favour of "d". The change started in High German and progress through the language.
This is why "the" in German is "die" and "that" is "dass" etc
Similar changes would have eventually come to English and it can be seen in where "the" is sometime replaced with "da" in some dialects. It is actually easier to say.
Such changes ended when languages were documented, inhibiting further change.
What we ended up with is a ridiculous system where spelling has little relationship to phonics and the rules make little sense so I wouldn't get too picky about what some people choose to do with pronounciation, sorry pronunciation. See what I mean?