Note that instances of most other types are non-subclassable "by accident".

>>> class A(42): pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: int() takes at most 2 arguments (3 given)

>>> class B:
...     def __init__(self, *args): pass
... 
>>> class C(B()): pass
... 
>>> C
<__main__.B object at 0x7fdcfb49aae0>

It is okay until we decide that there is a problem, and it that case it would require more general solution.

Are there any issues with this in real code?