Alright I see, Nintendo should put more into an effort of character development
The best way to do it is to avoid the character development being contrived. That could be pretty difficult. It's easy for anything involving space to become contrived, as it is a frontier we can't actively explore ourselves as humans yet (beyond observation). Other than that, I don't think seriousness is the way to go either. A little seriousness from time to time can go a long way, but when something is serious all the time it can come across as just superficial and thus less relatable.
There is a similar-but-different downside vice-versa. It's difficult to accrue a wider audience if you're making something too silly all the time. Sometimes, it feels like Star Fox dips too far into the silly side of things. It can work at times, of course, but the sense of balance can become muddied and thus less relatable.
It feels like some titles and franchises work better by being silly, while others need that balance between silly and serious (some leaning silly, some leaning serious). I would argue that Pikmin and Mario are two franchises that benefit from being more silly (with the exception of the Mario RPG and Paper Mario games). Metroid on the other hand feels like a franchise that works when it leans more towards serious. It's just the tone and set-up of the franchises, respectively.
Star Fox on the other hand almost feels out of place -- almost like a lost sheep. I get that the characters are "cartoony" anthropomorphic animals. That's why it works to be silly at times. At other times however, some people might wish that it had more to offer.
This is why I think the "real" argument is brilliant for Star Fox. Instead of shifting more towards serious, it might work out better for Star Fox to present the characters and their universe in a way that feels real. It does not need to be "realistic". It just needs to feel real as a universe. To achieve that level of care among the people who play the games would be an impressive feat.
One of my greatest fears for Star fox as a franchise is just how thin a line it potentially has to tread on if it changes. It's extremely difficult to predict how the series will fare if they even make such a change. There are many questions. How would the older fans of Star Fox respond to such a change? How can we be sure that they could nail such a thin "Goldilocks Zone" for balance between writing and gameplay? Is it even possible for the franchise, given its set-up and design, to accrue a larger audience? What other adverse effects might result if they make such a change? And so on...
I'm sure many of us want to see Star Fox become something bigger than what it is. I'm sure many of us see the possibilities for the series. That's part of the reason why we're fans of it.
The good thing is that nothing ever truly "ruins" something. Everything can change, evolve. The only thing that really "ruins" something is our perception of whatever it is we disliked or put us off. We can, albeit sometimes subconsciously, change the way we feel about something based on our perceptions.
Sorry for the wall of text.