I get extra slots taking more resources and creating longer load times for the screen, but we're talking about a cap. It's changing a maximum value which would run through a database the same way whether it was 200 or 999. Both are three-digit numbers, so tabulating wouldn't change resource usage. Maybe Bungie really has a vault somewhere and actually physically stacks 200 pieces of relic iron into piles with my name on them? If so I totally get their resistance to raising stack sizes.

Computers don't run on base 10. As far as a computer is concerned 200 is an 8 digit number.