Buying the forest doesn't allow you to go back to the rats, making it impossible to progress further since you don't farm enough.
The forest is actually the last part of the playtest, we haven't balanced anything beyond that! Thanks for playing to the end. We're going to release a longer demo soon on Steam! The demo will have further upgrades past forest.
We will definitely add in the ability to switch back to Alleyway, per your feedback!