That is definitely a strategy we considered, and there isn't really anything in place stopping that other than the bosses giving you an extra permanent upgrade so there is some incentive to move forward. We had a few ideas for how the upgrades should work, including buying them with a points system, but we settled on what we have because we felt like it made the most sense for a game jam. People might only play the game for 5 minutes, so what can we do to optimize for fun, and what can we do that would make people want to play for longer? If we made it into a bigger game we'd probably do something else, but as it stands if someone is having enough fun with the game that they want to grind out more upgrades by dying after the first wave, then I'm honestly flattered.
Thanks for playing, and for your thoughts!