The nature of spoken language is that of constant change. As has been mentioned here, regional dialects are different from each other and interactions with other regions accelerate changes. There is no way around it. There are many different dialects , even in davening among seemingly homogeneous groups. A Californian-born yeshivishe person sounds different than a Brooklyn-born yeshivishe person. Al achas kamah v’chama among populations from very different backgrounds.
Therefore it is pointless , even ridiculous, to discuss the “correctness” of one dialect over another, or one “havara over another”. All, every single one of them, from Teimani to Unterland,from Brooklyn to Buchari is legitimate, that is the nature of spoken language. And yes, even “litvishe yeshivish ” and “dati leumi” are equally legitimate. Ooo or eee, sss or ttt, ohhh or oyyy or aiii, they all count. Daven the way you daven, it is ok.
To illustrate the point, the gemarah in brachos requires the elongation of the “dalet” (do you say “daled”? It is “dalet”, but that’s ok!) at the end of the word “echad” in shma. It is a time-honored tradition to attempt this. However, no Ashkenazi or North African is capable of accomplishing it, as the dalet begins and ends abruptly, and what is actually elongated is the kumatz of the chet, as in “hashem echaaaaaaaaaaD ! So we are fooling ourselves into thinking that the dalet is elongated. There are dialects which pronounce the the dalet as a vibrating “th” sound, which can be elongated. So are ashkenazim reciting shma incorrectly? No, because dialects change and there cannot be a single unchanging one, nor has there ever been.
To the language experts out there, I apologize for using incorrect terminology, I myself am not a linguist.