I heard announcements (at the station in Tokyo):
この電車は桜木町行きです /
この電車は桜木町へ行きです
(not sure, did I hear correctly?)
English version of the above:
This train is bound for Sakuragichō.
So, I am asserting that we can use verb stem + desu. Is my assertion correct? Please explain.
No.
In your example, that 「行き」 is actually a nounified(名詞化) version of the verb stem of「行く」.
「桜木町行き」 translates to "One that goes to Sakuragichou" or "Sakuragichou-bound".
The 「です」 is not required for the nounification. That applies for any other instance of verb-stem nounification. (i.e. うそなき- fake crying, すなあそび- sand play)
This 行き isn't verb stem but a noun. Verb stem + desu is incorrect because a noun or attributive form of verbs and adjectives + の are placed before です.
連用形(continuous form) of verbs often become a noun.
In addition, この電車は桜木町へ行きです is unnatural but この電車は桜木町へ行きます is natural.
