Description
When configure your busses it could look like this:

framework:
    messenger:
        default_bus: messenger.bus.command
        buses:
            messenger.bus.command:
                middleware:
                    - App\Message\Middleware\SpecialLoggingMiddleware
                    - validation
                    - doctrine_transaction

            messenger.bus.event:
                default_middleware: allow_no_handlers
                middleware:
                    - validation

            messenger.bus.query:
                middleware:
                    - validation

Since we are using multiple busses we are forced to define default_bus. Why?
I understand it is for autowire but what if I dont want it to autowire?

The issue is the following:

I define my services like

services:
    _defaults:
        autowire: true
        autoconfigure: true
        bind:
            $eventBus: '@messenger.bus.event'
            $commandBus: '@messenger.bus.command'

So if I create two PHP classes like this:

class Foo{
    private $eventBus;
    public function __construct(MessageBusInterface $eventBus) {
        $this->eventBus = $eventBus;
    }
}

class Bar{
    private $queryBus;
    public function __construct(MessageBusInterface $queryBus) {
        $this->queryBus = $queryBus;
    }
}

Foo will get an instance of my messenger.bus.event and Bar will get an instance of messenger.bus.command. You would expect Bar to get messenger.bus.query, but I've forgotten to add that under services._default.bind and autowire "helps" me to get the configured default_bus.

I would much rather not define a default_bus and then get an exception since it cannot autowire MessageBusInterface $queryBus.

Im sure there is a reason why we need a default_bus but I cannot see it.