Well yeah his plan changed after she died. Originally he had planned to go back and help her fight but after she died, he knew there was no going back to that point in time even if the time machine was still working. They explained this earlier in the movie.
What’s the point of coming back to help her fight... when part of her issues as an adult where that he left her alone and died...
Like his logic at the scene at the school about he reasons to track down the aliens to save his daughter made sense... then cause they would be saving his daughter from that future
But to say.. I’m going back in time to create this serum to then go back to the future with this serum to kill these aliens to save a future that’s already fucked...
makes zero sense from a time travel standpoint because they knew already when the aliens really started attacking and they knew specifically where they came from...
They knew they came from Russia.. They knew they attacked in full in 30 years..
So now you have a serum... And an entire world looking for hope to stop this shit..
You can’t tell me.. that coming from the future with ... We have a weapon to defeat them.. we have 30 years... that that shit wouldn’t get people to prepare for this war..
Like if they tried and it failed... that’s what the future people are there for.. they could have easily said..
“We had a weapon and it failed” so drafting you is what we have to do...
Also Old girl was working on the cure before the aliens attacked at the base ... the same base with the time travel machine.. wouldn’t it have made way more sense.. to send some of her research back to
The past with some of the other soldiers while she was working on it as a fail safe.
Like you have a time machine... aliens start breaching the shit.. you send somebody back..
Like Chris Pratt was already going back because of the time limit but you couldn’t send other future people back.