That's what I have trouble with. E.g. the climbing rat would be able to get all these higher food sources on trees. But evolving wings loses the ability to climb. And now it's competing with faster and more sturdy rats on the ground again.
I can only think about the disadvantages being no real disadvantages, because there is abundance of food and no serious competition/predators.
Evolution will try everything. If there's sturdy carnivorous rats around, the rest of the rats will try their niche. Some will try faster, some will try playing dead, others will dig holes to hide in, or climb trees. Some might grow thicker skin, or looser skin, or venom, or just being poisonous to eat.
These empty niches can be filled pretty quickly. After removing top predators like wolves for instance, the average size of coyotes has been steadily increasing over the last 50 years.
A side note, plants figured (in the evolutionary sense) that birds were great seed spreaders compared to the non-birds. Some evolved impressively strong spices (Capsicum) that birds are impervious to (up to several % by weight) that eliminated non-bird consumption of their berries. So to protect bird feed from squirrels at Capsicum.
Any bird like feature will help with a particular niche. Feathers, beaks, dexterous claws, bird songs, gliding, and of course powered flight. So it's not like you need a huge jump from land based mammal to full flight before you have any evolutionary advantage for a particular niche.
I think you are skipping steps as you try to imagine the process. Wings are at the extreme, there are intermediate features. Checkout a video of "flying squirrels", you will notice that they are more like "gliding squirrels".