It's for sure isn't explained well enough, and that's on me. But when you scale the enemies, you are given more coins for beating the level. Thus increasing your score (which is sadly on displayed at the end) and giving you more coins to spend on yourself.
Yes you can upgrade your jump and speed so high that you can just jump to the end, but you'll also get a low score.
Alternatively, you can hard level health/damage/defense, and slowly go throught the levels and get a high score.
To beat the game, and get the highest score possible, you have to make all the enemies 10 in all attributes, and in order to survive you need at least 4s in all player attributes, then kill all enemies(except birds. I forgot I never gave birds health). and collect all coins.
There's also a bug where your(and the enemies) stats get multiplied before each level, instead of ONLY when you level them up. So oops,