Short and unambiguous
Complexity makes a grammar hard to learn, but not hard to use. Speakers of a language, having already learned the language, are not really interested in
reducing its complexity. It is also not a conscious choice to adhere to the language's rules, so they don't feel "restricted" by them.
What an language speaker wants from their language is the ability to get their point across accurately and fast. So a grammar has to reduce ambiguousness while still allowing to form short sentences. That is what they need for "communication purposes".
If we measure complexity as the number of rules a grammar has, we see, that:
We need rules to make sentences less ambiguous: Forcing the word order to be SPO or marking cases on nouns gives an unambiguous way to parse a sentence.
Adding more rules to a grammar can also make it less verbose.
to be being irregular in English allows for the shorter
But making a language less ambiguous also entails forcing speakers to give some extra information:
For example, you can drop the subject in Japanese and it has to be understood from context, sometimes leading to misunderstandings. German and English force you to specify the subject to counter this possible ambiguousness.
This, on the other hand, makes the language slightly more verbose.
Most grammar rules I can think of reduce either verbosity or ambiguousness.
Regarding gendered nouns:
German forces its speakers to mark the gender of almost every person as soon they are mentioned by using gendered nouns. So German speakers, in return, think it is very strange to hear from "a teacher" and not know if they're male or female.
This may not sound so useful, but gendering nouns comes at a very low cost in German: it is mainly marked on articles and adjectives that are already inflected for case, and the inflections are very short and easy to pronounce.
The biggest benefit of gendered nouns is that it reduces the ambiguousness of demonstrative, relative and personal pronouns,
because only nouns with the right gender could be meant.
In the sentence
Das Pferd ging in den Stall und aß eine Karotte. Sie war orange.
The horse went into the stable and ate a carrot. It was orange.
the it can only be the carrot, not the horse. Using the other pronoun "es" we could have made it the horse.
To summarize: Complexity is not really a problem of a language's speakers,
and adding more complexity can reduce ambiguousness and verbosity, which bothers speakers the most.