I must disagree with your Decimal datatype claim that "unfortunately, it was never actually implemented." It and the scaled integer datatype, Currency, are actually variant objects that behave as a particular datatype. As such, they don't perform as well as the intrinsic numeric data types.
Try the following code snippet:
Dim x As Variant
x = CDec("12313155586486444894668797665")
Debug.Print TypeName(x), x
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