I know this comment is really old but i just wanted to answer it for some reason
Well, its not only TD, it is also idle, and in idle main point of the game is to make progress and to make progress as fast as possible. Also idles are often do not punish you, slow progress is punishment by itself. To be honest i kinda like the idea of idle with some kind punishment, but i think in this game i handled it pretty well still. First, missing enemy often makes your turret rotate excessively and in general makes everything more chaotic, making your "defense" much less effective, so even though i know there is no punishment, i myself still try to avoid enemies looping when i play the game. Second, you can accidently spend too much money on upgrading enemies, making them impossible to kill with just one loop. So in case i added some punishment, people would end up in infinite cycle of losing, and so i would have to change the whole concept of the game to somehow prevent it (and i dont like that). Thirdly, im not sure what do you mean by using speed enchantment with no risk, upgrading speed too much makes enemies much harder or even impossible to kill if you dont have spikes or lasers, so the added reward for killing the enemy will be useless then. Whole point of speed upgrade is for you to balance added reward and added complexity to kill the enemy. And well, it works for all upgrades, thats like the main point of the game: balancing defense/offense. Not sure why you mean speed upgrade specifically