Long-time lurker on the thread, but my thoughts have been:
Law of supply and demand. There were probably tons of guys who would've carried a PGA pro bag for a similar amount. The caddie hasn't made any case that he meaningfully contributed to the win by teaching Kuchar how the greens break, or maybe how certain shots play longer than they look due to prevailing winds, or whatever. What value did the guy bring that a bunch of others couldn't have?
A traveling caddie is 100% different. They invest themselves in the player's game, they commit to travel, and they likely do a ton more for their player due to the relationship than a local caddie. A standard (and higher) percentage of earnings is appropriate.
I don't really care if Kuchar made $1,000,000,000,000 or $100,000. It boils down to me to be: how many other guys could've and would've done the same task for the week? Probably tons. It's fair to pay people according to the value they bring- regardless of the purse that was won.
Kuchar can be charitable with his money in other ways to show what kind of a person he is. He likely has charitable causes that he contributes to. But don't measure how caring or uncaring he is based on this business relationship that is essentially driven by supply and demand.