Thanks!
You’re absolutely right. For 90% of the dev time, the upgrades did indeed alternate (wave 1 = new enemy, wave 2 = upgrade, wave 3 = new enemy again, and so forth). While testing, people felt the diversity in enemies was more interesting than the upgrades at the start, which is why I changed it last-minute to introduce only new enemies in the first few waves.
Then again, I was only able to find 2 people to quickly test the game. Maybe they just happened to like seeing new animals ;)