Wiring power banks in series for 25V output to power e-bike
Your power banks provide 5V 2A output or 10W, and cost about $10. To power a 250W motor, you'll need 25 power banks which will cost you $250. Why not just buy a $200 battery then?

Also, think how bulky those 25 power banks will be on your bike, and what a pain they will be to charge.

Hopefully these simple arguments are enough to persuade you, before I'll have to explain all the balancing issues you'll face once you connect Li-ion cells in series, or consider custom charging circuits you'll need to design if you remove the original PCBs.