Sorry, my comment was about it being slow, not about it being a bad idea: It is a bad idea, but it will not really make your python even slower than it is today. The Intel supports most of ieee754-2008 which has most of what you would need.
Judging from other comments here, it is not clear how widely supported IEEE754-2008 is. If it isn’t supported everywhere, it would make a VERY bad default for a numeric type.
It also appears that the logic for implemented something like this standard is indeed slower than standard IEEE754. Even if it’s only a bit slower, seems bad to make it the default.
All this just to fix something which is confusing to a novice programmer… and this is leaving aside the additional complications a fixed width decimal has which a floating point type doesn’t have.
It's fully supported. However, some parts only via a library. Anyway, to drill down my argument. It doesn't matter how slow or fast it is: python's floats ints,... are boxed. Your performance is down the drain before you even start looking at the value at hand. If you want any kind of performance at all while crunching numbers you will not be doing it in Python (which is exactly what happens when you use things like numpy)
https://en.wikipedia.org/wiki/Intel_BCD_opcode