Good points!

Many FP lovers refill from bottles, but it isn't necessary. Cartridges are easy to use, well-suited for travel when you may run dry in the middle of a trip, and TSA doesn't give you grief about them.

Again, my point to OP was that there's very little practical advantage to ball points over fountain pens; if you're using cartridges, they're not much better for the environment, but you can do things like use a converter most of the time and carry cartridges as back-up. Fountain pens are fantastic writing implements.