Thank you for playing! Yep currently I tried to explain the end goal in two ways.
1. The one is direct - kill the boss at the end of the level and return with diamonds - upgrade - try again, which is optional.
2. The second goal is set through narrative - when time doesn't exist anymore - human nature kicks in, we start to get greedy and want more and more. So greed moves humanity forward and to do that you need to collect diamonds. So just go and collect diamonds or just waste your endless time.
And for games that are completionists, I added an option to max out all upgrades.