Yes, I’ve done that (I call the type Num).
But without a basic type, expressions are cumbersome.
My example from a previous post: x = (a * .95) + b / 12
becomes
var a = Num(1234)
var b = Num(5.678)
var x = a * Num(.95) + b / Num(12)
which is far better than Java, but is still more verbose than necessary. Kotlin/Swift and others have gone to extraordinary lengths to get rid of a few characters here and a few characters there. For business applications, d128 would save lots of boilerplate for this kind of programming. This really is low hanging fruit in the quest for succinctness.
With a d128 basic type, the code would be:
var a = 1234d
var b = 5.678d
var x = a * .95d + b / 12d