Pretty sure his channel is successful due in no small part to his voice. It couldn't not be, considering the whole thing is just voiceovers. You also often see people commenting as such on his videos.
You have a point about "s" and "th" which is a whistling between tongue and palate.
There are also plugins that take care of at least most of it now, but it's not going to get rid of every instance of it. In current year, if you want 100% of it, it makes more sense to just train a model on your speech and fake all of the audio instead of recording it, then re-record only the parts you don't find acceptable.