I suppose in a very strict sense it may not be entirely wrong to substitute the WTS rendering of "underserved kindness" for the simple word "grace" The WT attitude of Freddie Flintstone Franz was: Why use one word to mean something when two or more can make it sound more pompous. Why make something simple when it is so simple to make it complicated.
The expression "Undeserved Kindness" is the expression that Franz used to render the Greek word "Charis" While it may not be inaccurate to translate the Greek word this way, it does not in any way make the translation any more accurate. I say this for at least two reasons:
1 It is'nt strictly correct to call "Charis" something that is "undeserved", because most lexicons affirm that the underlying idea behind the term is that of something being "Unmerited" - ie something that we have not, indeed cannot, earn.
2 The attribute of God displayed in the bestowing of "Charis" on us, goes beyond simply being "Kindness" It displays a deep love that He has for His human creation in that He is willing to grant them favours in order for them to become reconciled to Him. That is why lexicons describe "Charis" as a "Favour" rather than a "kindness" The Enc Of Bible Words [pg 317] describes "Charis'' this way: "It means a gracious favour or benefit bestowed"
Therefore, I submit that if the WTS Bible wanted to display a leaning to "accuracy" they would have been better off using the expression "Unmerited Favour" [The Amplified Bible suggests: "Spiritual favour and blessing", as does Prof Chas B Williams in his translation. The Simple English Bible has "Gracious Love"]
Anyway, there is a perfectly adequate one-word translation that we could use - Grace - which is why most intelligible versions do so.
Besides, as lighthouse 1956 has so eloquently pointed out, "It's so much easier to sing about Grace!!"