Parasha question for the week: Beshalach

[Alex]

After having witnessed the 10 plagues and the 10 miracles at the sea (Avos 5:4 cf. Rav there), Our ancestors sang.
HKB"H did not even permit the angels to do this, as it would seem to celebrate the destruction of creation (Megillah 10b).
The Torah states that אז ישיר. The future tense is used here, so it could read "Israel will sing". There are a ton of explanations for this but, really, there is no sense of finality to the song. When did the song start? Will it end?

Essentially, What do you sing for?