It would take quite a lot of effort to offer the new print function and the old print statement at the same time, and it would require either a lot of special cases or renaming the new print function.
And there's really no upside. If the only legacy thing in your script is the use of print statements, that's trivial to fix by hand or with one of the automatic converters.
Yes there is, the print function is the biggest syntactic incompatibility. Besides the (optional) u marker for unicode strings that they reintroduced later. Most of the other difference (like changed packages) are semantic.
You would have been able to import and run Python 2 in Python 3, maybe with a couple of "if"s or some magic in the module loader. Breaking compatibility like this felt petty and deliberate and made it really hard to have one codebase target both python 2 and 3 (temporarily).
But as the GP mentioned, it can be fixed automatically.
In this specific case I think it's a non-issue. If python3 was limited (or limited except in very niche cases) to changes that could be made programmatically, we wouldn't be in this situation. Everyone would just run 2to3.py and never look back.
The problem here is that print "statement" can't be upgraded to a log "function" easily as the programmer grows in experience.
A print "function" conversely is quite easy to upgrade to a log "function".
In addition, "print" is almost always a code smell in professional code. The moment you need to localize, change destination, etc., "print" goes right out the window.
I have lots of beefs with Python 3. Changing print to a function is NOT one of them.
It would take quite a lot of effort to offer the new print function and the old print statement at the same time, and it would require either a lot of special cases or renaming the new print function.
And there's really no upside. If the only legacy thing in your script is the use of print statements, that's trivial to fix by hand or with one of the automatic converters.