^I think part of the problem with 100 Park Avenue's facade was what it was made of- brick. Over time, many of those facades didn't age well and developed maintenance problems, which were probably getting expensive to deal with, so something needed to be done. Remember what 425 Park Avenue used to look like, same architects and same time period.
That and a modern facade was probably thought to be more appealing to potential tenants. I do agree it was nice looking before though
