People like to use software they can trust. Having an out-of-date UI is a violation of trust. It says that whoever owns it doesn't care about it anymore.
In comparison, I'm still using foobar2000 and Craigslist 20 years later.
Other things like non-monetized blogs can stand being retro.
I'm sure PMs and Data Analytics teams have actual data to back me up on this, and if the data shows to the contrary please correct me. I'm working with a small subset of friends and family who are, for the most part, your "average" user, so it's all anecdata for me.
On the flip side, stop fucking with messaging app UI.
I don't care about a splash of CSS to make it look fresh - just leave the rest of the UI alone, please.