The trope is correctly placed because whether the accent falls on the first syllable (where it belongs) or even on the last syllable, there is no grammatical change to the meaning of the word.
However, if the accent were to fall on the penultimate syllable (second from last) then the word would change its meaning, and this occurs in Exodus 10:26.
The accent on the penultimate syllable forces us to read the suffix -nu as a first person (plural) possessive adjective, and therefore to read the word as noun (or an infinitive, which is a verbal noun).
In other words, when the Hebrew letters באנו appear together, the word can have one of two meanings depending on where the accent falls. If the accent falls on the first (or even the last) syllable, the word is the qal perfect (first person plural) of the verb בּוֹא. However, if the accent falls on the penultimate syllable, we are forced to read the word as the infinitive construct, but now the suffix becomes possessive, and the word is now understood as a verbal noun (our coming) instead of the simple verb in the first person plural (we came).
Finally, the cantillation of the verse (based on logical arrangement) appears as follows.
Please click to enlarge.
Please note that the word בָּאנוּ appears early in the verse, and marks off the principal division of the first half of the verse. But there are also several subsequent logical divisions of the verse that follow. Each of these divisions (and subdivisions) are marked off by disjunctive accents, which CANNOT be stronger than the accent assigned to the word בָּאנוּ until we arrive to the mid-point of the verse, which is marked by the Athnach.
Since our word in question marks off the first logical division of the first half of the verse, it must carry the most powerful disjunctive accent until we arrive to the Athnach, but that disjunctive accent cannot have its stress on the penultimate syllable, otherwise the grammatical meaning of בָּאנוּ will change (for the reasons already mentioned in the paragraphs above).
The ONLY disjunctive accent that can fit here, and that does NOT have the accent on the penultimate syllable, is Segoletta. (The next qualified disjunctive accent is the Zarqa, which is too weak, not to mention that the Zarqa had already appeared on the word preceding בָּאנוּ֒.)
In other words, the remaining disjunctive accents carry their stress on the penultimate syllable, and such use of these accents would change the grammatical meaning of the word (for reasons already noted ). For these reasons, the Masoretic editors had to use the Segoletta, which is postpositive (and so falls on the last syllable), and therefore does not change the grammatical meaning of בָּאנוּ֒.
