This company has been in business for over 250 years. The OP stated his pen "Has fallen apart", without giving any details as to how this actually came about. As to Faber Castell refusing to carry out repairs unless their ink was used does stretch the bounds of credulity to a high degree, unless of course proof can be shown that the company did actually state this.
What we are getting here is individuals jumping to conclusions just on statements being made without showing facts. This would never be admissible in a court of law.
Well, you're lucky that it has never happened to you.
I'm inclined to believe the OP because this has happened to me albeit on a pen from another manufacturer and it was graciously replaced even though the pen was way out of warranty, no questions asked.
My point is that, there should be a degree of trust between the manufacturer and the buyer because if someone cares enough to buy their products, manufacturers should at least acknowledge and express concern that the issue happened at all. The response from Fa-Ca sounds like they don't care much ("after reviewing it they might replace the section"). To me, the response should've been,
"it is unfortunate that our product did not meet your expectations. We will gladly replace/repair it for you after you send in the faulty product. We will investigate the matter further to ensure such issues don't happen in future."
I think it is fair to assume that a company that has been around for 250 years is willing to stand by their products and understand that such things can happen. A few bucks of goodwill would keep the customer happy and they are more likely to remain a customer. Of course if just a couple of people are intentionally damaging the pen and asking for replacement every couple of months, then that matter can be handled differently. Here, that doesn't seem to be the case.
And the thing about using only their inks, we all know why that is so.
I don't believe that being in business for 250 years is an indicator of good customer service or maintaining same level of quality in the fountain pen world. The quality on their high end GvFC pens might be high but we're talking about cheap entry level pens. Take Parker, for example, their Duofolds are great pens but the Vector is a hit or a miss.