A 'black hole' for data! Oh, that made me laugh. We have a great argument-stopper in our household. When someone comes out with something along those lines the challenge is "Name three." (That is, name 3 times you did that, name 3 examples of when that happened, etc. and if they can't respond immediately then they are deemed to have lost the argument.) So, my challenge to the LAMP guy is "Name 3 examples they know of where Filemaker has acted like a Black Hole."
Let me guess that the same employee is well-versed in LAMP technologies, and not that well-versed in Filemaker. So, amazingly, their services will be much in demand in one scenario.
Filemaker's ability to be used for rapidly application development and the user-friendliness of the interface is legendary. Of course the functionality and user-friendliness is only as good as the developer makes it - just as in any LAMP application.
"The LAMP environment would allow us to implement far more feature than Filemaker Pro offers" - Name 3, and we'll discuss.
"including a literally unlimited choice of user interfaces" - And Filemaker only comes with a handful?
"This degree of customization will negate the need for end-user training as the interface design can be greatly simplified compared with FileMaker's" - If the interface is designed badly it will take much training and generate frustration no matter what the platform. And if it is designed well it will be an intuitive breeze to learn. The argument is redundant.
I'm not a Filemaker Sales guy, but you are on a Filemaker Forum, so I suppose you wouldn't have expected anything else than a defence of a platform that is used in millions of applications from Blue Chips to one-man-bands.