Simple reasons why Imu will win (meaning he will not be defeated and at least cause substantial damage)
1. The final villain needs to be a real threat and show his power
2. Strawhats need a Sabaody-like defeat after timeskip to get power-ups
3. Perfect transition to Ragnarok and escalation of global war arc (just like Madaras first appearance)
4. Luffy has not mastered awakening and Kings haki and needs to do so to defeat Imu + Luffy was always defeated by overpowered villains first
5. The mural teases Imu on the left side being attacked by what is supposed to look like the Strawhat pirate alliance. This shows the real battle against Imu, which will not happen now. Still enough mysteries and stories to be solved before Imus defeat (Lily, Vivi, 20 kings, void century, Mary Geoise incident, mural, etc.)
6. If Imu dies, most of gorosei and probably holy knights will die with him, causing immediate collapse to the WG (which will be dissappointing lol)