The PEP itself in "rejected" ideas makes an argument against it:

indented stuff after a line ending with ":" must be a *statement*. It
would be totally nuts for that to be something else, e.g. an expression:

That of course leads us to the obvious idea:

match a:
case 1:
     ...
case 2:
     ...
else:
     ...


Of course, PEP smartly has an argument against that too, in the vein of
"after line ending with ':', there should be an indent suite (list of
statements)". But that's where it goes sideways.

Exactly. I think not identing what comes after match a: comes with far more advantages than disadvantages. In fact, if the only "real" issue is that anything after : must be indented, I'd say it is vastly outweighed by the issues not doing it would solve.

Would the parser allow us to write the above without the colon, thus avoiding the aforementioned objection?

And if the above snippet looks weird to anybody, it's only because of
all the "case" business. There wouldn't be such a problem if it was
instead:

match a:
| 1:
     ...
| 2:
     ...
|:
     ...

The above ML-like syntax should be perfect for almost everyone, ...
except the PEP authors, because they have it in "rejected ideas" either.

I don't know, that doesn't look very pythonic based on the preference Python makes for words over symbols. I don't think there's any problem with using case, it's the expectations many have for what should come around it that creates issues.

I can't think of one at the moment, but I don't think you
should dismiss tradition so easily.

I don't mean to dismiss it, but I wouldn't want it to get in the way of a change if there are good reasons to go against tradition. This seems like one of these cases to me.

One of the arguments used
to justify significant indentation in Python is that "you're
going to indent it for readability anyway, so the compiler
might as well take notice of it".

For the most part, Python indentation follows what people
would naturally do even if they didn't have to. So I think it's
worth looking at what people typically do in other languages
that don't have mandatory indentation.

[...]

or like this:

   switch (x) {
   case 1:
      ...
   case 2:
      ...
   default:
      ...
   }

This suggests to me that most people think of the cases as being
subordinate to the switch, and the default being on the same level
as the other cases.

I think that last one would be perceived as ok if it weren't for the brackets: everyone naturally indents any set of brackets' contents. I wouldn't normally mind cases being indented extra relative to "match" if not for two points:

1. Python relies heavily on indentation. It's likely for a match block to appear in a place that is already well indented, *and* it is also likely to often include further levels of indentation. It would be preferable not to require two levels at a minimum every time a match block is needed.

2. Unless the default case is signalled with an ad-hoc keyword like default (which I personally would like), it seems a wildcard symbol like the controversial _ is needed to pair with case. Going against tradition and setting only one indentation level would solve this issue, while still allowing (I think) any programmer to indent the default case extra if they so wish.

If none of this is convincing, I do believe the matter is easier than it appears, because few would be confused by else being at the same level as each other case.