Sing Again

On the words אז ישיר משה, Rashi in his second interpretation says the reason why ישיר is in future tense is אבל מדרשו: אמרו רבותינו: מכאן רמז לתחיית המתים מן התורה.  According to this derash it means that when the dead will be revived, then אז ישיר will be sung again.  Why, it is long gone and done?
Dovid Hamelech in Tehillim (30) says ה' אלקי שִׁוַּ֥עְתִּי אֵ֝לֶ֗יךָ וַתִּרְפָּאֵֽנִי.  What does Dovid Hamelech being healed from his sickness have to do with building the Beis Hamikdash?  The Even Haezel in his intro. to Kodshim 2 explains that the nature of a person is that if s/he gets sick then all they are thinking about is getting better.  If they are cured, then they give thanks for that.  In they are successful later in life and gain wealth and fame, they have a separate gratitude to Hashem but the thoughts of their days in their sickness are long gone and they no longer are feeling appreciation for surviving.  However, Dovid Hamelech wasn't that way.  He didn't merely sing the praises of Hashem for bringing him from the previous stage to his current stage, but he saw the entire picture.  He gained a deeper appreciation for Hashem helping him out in his times of great need when he can now, later on see a fuller picture and how this led to preparing the Beis Hamikdash.  At this point Dovid Hamelech thanks Hashem again; not a repetition of the old thanks, but as a new message for now he has a deeper appreciation of where being saved when he was sick led too.
This is why אז ישיר will be repeated in the future.  Of course, משה and Klal Yisroel were very happy to survive the Yam Suf.  However, the full effect of this momentous event can only be appreciated in תחיית המתים when the full subsequent string of events and how it was necessary for the final tikkun of the world will be realized.  At that point a new אז ישיר is necessary for they will have a deeper appreciation of the miracle of Krias Yam Suf (based upon וענפיה ענפי ארז.)