Of course both Wei Wuyin and the old man were well aware that spiritual strength wasn't all-powerful, as there were numerous other factors that determined strength. The key and most crucial being cultivation level. Even if Wei Wuyin's spiritual strength was terrifyingly strong, what was that before the Worldly Domain of a Realmlord?!
A Timelord's Temporal Dissonance?!
A Starlord's Star Implosion?!
Could Spiritual Strength overcome these innate qualities of a higher cultivation level? Perhaps, but this didn't factor in the level of cultivation methods, exquisite spells, grand formations, bloodline abilities, and so much more external factors, such as armaments, talismans, pellets, etc.