Multi-cell LiPo charging from 5V USB, without boost circuit
The problem is that transistors are not exactly switches, and they especially do not provide isolation between the sides. If you assume the top cell is "on" and its lower terminal is at ground, the lower terminal of the bottom cell is at -5V and the transistor is reverse biased. Their voltage tolerance for this situation is usually quite bad, and it will also be difficult to turn it on. Have you tried building this in Falstad and simulating it? What happens?

(The whole thing seems considerably more complicated with higher part count than just building a boost converter and balancer?)