I definitely agree with you for tuples especially smaller tuples. As tuples get larger however performance can swing the other way. As someone who codes some F# for my day job it seems that while tuples should be easier in a functional language like F# for backwards compatibility struct tuples will be more awkward to use than in C# 7. I think they should of just bit the bullet for tuples up to a certain size.

For records and unions however I think the attribute is fine. Most of the time for types of more than 3 fields say structs can be worse in performance than classes anyway. Records and unions tend to be assigned more and live longer than tuples most of the time. For unions for obvious reasons structs are not recursive in nature which with unions is required somewhat (see https://msdn.microsoft.com/en-us/library/ms229017%28v=vs.110...). Definitely wouldn't want structs to be the default here.