LINQ is indeed optional, but this discussion is about the evolution of the language, with LINQ being held as an improvement. And because it is new and shiny, C# code across the globe quickly became infected with it.

Your comment on "where there is no noticeable performance hits" strikes a chord because when first used there are no noticeable performance hits. But then that application grows and scales and suddenly it is death by a million paper cuts, thousands of grossly inefficient set operations devastating performance. That's aside from the fact that LINQ is often a short-circuit saving from having to think about appropriate algorithms of object-methods to deal with the likely uses.