During the last scene in Sunday's boss fight, he asked "Why does life slumber?"
To which the Trailblazer answered, "Because someday, we'll wake up from our dreams"
TB's answer may have caused confusion to a lot of players, as it did to me. But despite not knowing what it really meant, it really resonated with me, so I wanted to find out more as to what TB meant with their answer.
After pondering about the dialogues starting from the beginning of the penacony quest to present, I was able to piece two and two together and roughly understand the meaning on their answer.
I think what TB meant by “because someday, we’ll wake up from our dreams” to his question “why does life slumber” is that: it’s metaphorical. it means that life, in moments of quiet and dormancy, is akin to slumbering.
“why does life slumber?” - can also be interpreted as "why does life have periods of dormancy?"
We all know that Sunday’s motives/reasons on wanting to keep everyone in the dreamscape was because he realized that whatever he tries to do, he won't be able to solve everyone's problems nor can he save everyone. Everyone has their own suffering that they cannot escape, hence, he wants to offer everyone the dreamscape where they can pursue eternal happiness (even if it was a lie) and not leave. He saw that no matter how desperately he tries to do the right thing, all of his efforts were always met with failure. He was raised with the mindset that he must help everyone no matter what and alleviate their suffering under the watch of the Harmony. But as he grew up, he realizes that the Harmony CANNOT give him the peace that he wanted, the strong wouldn't be able to protect the weak. So he abandons Harmony, and pursued Order.
“because someday, we’ll wake up from our dreams” - can be interpreted as "someday, we will emerge from dormancy and stagnation"
TB’s implying that these dormant times are like a dream and that we will awaken from them eventually. TB is suggesting that these phases are temporary, and eventually, we will emerge from them just as how we wake up from our dreams. We slumber (close our eyes, dormant) whenever we’re tired, unmotivated, and whenever we don't want to face reality... but yet, we don't want to give up either... we don't wanna die (permanently slumber) just yet. and "death" (to die) is to completely give up our dreams and ambitions, and surrender to our sufferings in defeat.
Hence: “sleep is a rehearsal of death”. ultimately leading to “we sleep because we aren’t ready to face death”.
and as we sleep... we dream. (delusion)
"Why does life slumber?"
"Because someday, we'll wake up from our dreams"
- life slumbers (is dormant) because it's waiting for us to wake up from our dreams (delusion) [snap back to reality]…
- while we're indulging ourselves in a beautiful dream, life sleeps - waiting for us to wake up...
I think this is the message TB meant to say to Sunday, he's so obsessed and caught up on wanting to "bring happiness" to everyone that he doesn't even realize that people just refuse to wake up on their own accord. He needs to know that the suffering that we all go through is inevitable, and deluding yourself with a dream without addressing your problems will never bring you true happiness. Again, this is just my own interpretation. I might be wrong or whatever, but I just think it'll be nice to share these thoughts 
