Bottom line, the UT is completely unrealistic, but who cares? We know that it is just a plot device, because it is a TV show and you can't have everyone speaking in all sorts of made-up languages.It's no problem, we can use the suspension of disbelief just fine, and it still sounds less stupid than the idea that everyone in the galaxy is speaking English.
Yeah. And anything more realistic would be incredibly clumsy. Some degree of realism can be - and has been - done short-term such as for a movie or for a sentence here and there in Trek, but a TV show, even just TOS? Nah. For me, "Universal translator" is merely shorthand for "somehow they bridged the language barrier, but we're not going to show you how because it would get in the way of the plot. So let's get back to our story, OK?" It's as simple as that.