This issue isn't unique to Cussler novels, but sometimes when reading a book or watching a movie, the ending is almost a bit of a letdown for me. Some of the earlier scenes in Treasure were so great that the ending didn't seem so climatic to me.
Cussler's setup was perfect with the hundreds of thousands crossing the river and advancing towards the treasure. Cussler seems to like to use the theme of the one-on-one confrontation finale and that was used here.
Today I was imagining an alternate ending:
Instead of Pitt wearing the Roman outfit and Topiltzin approaching... The illuminated Pitt was disguised wearing a mask and Aztec robe as he delivered a powerful speech amplified by loudspeakers. Lily was hidden underneath the robe at Pitt's feet, translating the words for him to deliver in the ancient dialog. The trick appeared to be working as Pitt's godlike speech was stirring the crowd into a frenzy. Topiltzin was furious that an absurd hoax was winning the confidence of his followers, but was powerless as the amplified Aztec voice drowned out his own commands...
Then Ibn appeared out of nowhere onto the platform with a knife lunging at Pitt. Pitt had to fight him standing almost completely still since he could barely move with Lily hidden under his robe. The crowd witnessed Pitt land the perfect punch sending Ibn flying off the platform. As Pitt delivered the final words of the speech ordering the masses back to Mexico, a stampede began and Topiltzin was trampled by the thousands rushing back toward the river.