* "It makes it more complex"
* "It's harder to follow the flow"

Depends on your measure of complexity. For me, I prioritise "area under
the indentation" as my preferred complexity metric (more lines*indents
== more complex), as well as left-to-right reading of each line (more
random access == more complex). By these measures, ?. significantly
reduces the complexity over any of the current or future alternatives::

def f(a=None):
     name = 'default'
     if a is not None:
         user = a.get_user()
         if user is not None:
             name = user.name
     print(name)

def f(a=None):
     print(a?.get_user()?.name ?? 'none')