1. More Native Americans live outside the reservations than inside.
2. The life within the reservations has been compared to that of a developing country.
Should they just be abolished?
The short answer is that the reservations are their land, they are the sovereign remnants after centuries of expropriation, abuse, and bad policy, and any decision about what happens to them lies with the people who have sovereignty within the reservations. Also, while many have serious poverty, there are some whose sovereignty and nearness to cities have been an advantage, as in Phoenix.
But as a thought experiment, what do you envision abolition as involving? Let's assume that the governments within the reservations decide, ok, let's give up any sense of independent government. What would happen next in terms of land ownership, government, etc? How would it help the people who live there now?