Hm, I didn't even know this is possible ;)

Would you agree on fixing, but deprecating this and include it in a bugfix version? As this is what I would have done for 5.0.0 if I would have know about it.

It should be fixed for 5.x but I wouldn't go as far as deprecating it since that should be done in TYPO3 core first then.

but I wouldn't go as far as deprecating it since that should be done in TYPO3 core first then

Not sure why. I deprecated things like using options as arguments and vice versa in console, although the TYPO3 does not deprecate it.
I did so, because I think it does not make sense to mix both in the long run. There is no benefit but just confusion in supporting both.

This is similar. How do you think it is beneficial to convert a returned string to output?

It is just a different way of doing things.

return 'Here is my final output';

vs.

$this->outputLine('Here is my final output');

I don't see how implicit is any better than explicit.
On contrary. You need deep inside knowledge of how command dispatching works to understand the first, while the latter is directly "documented" in the CommandController API.

It's not really limited to CLI, the same works for web requests and always has in Extbase which would lead to an inconsistency:

1. Returning strings in web controller actions is fine.
2. Returning strings in command controller commands is deprecated.

We should keep this in mind too.

It's not really limited to CLI, the same works for web requests and always has in Extbase

Imho one of the major flaws in the CommandController concept is exactly that it tries to mimik
a web request, while it is a totally different domain. Web requests have no interaction, so there won't be any output when a request is running and of course no input.

With the integration of symfony/console into command controllers (long time ago and also in Flow) the previously existing view logic was basically made obsolete, as symfony just directly prints out everything directly.

So yes, command controllers are different, because the domain is different. Trying to apply concepts from web requests to the command line does not help.

And: you can change your command controllers to use $this->outputLine() and it will work with or without TYPO3 Console. It would even work in TYPO3 6.2

Sure thing (and I'll fix my command), just wanted to make sure we don't forget anything here. Personally I'm fine to use the (superior) output API.