but we still have GA planes that are 40 years old and the designs are 60-80 years old
It brings a smile to my face to when I see cigarette lighter port is in the cockpit because that means the plane is from the days when not only smoking was allowed in flight, but the crew needed to smoke.
I don't think there's really an issue with the old designs if they've proven themselves over the decades, but the bigger issue is that a lot of these airframes are edging towards service limits, new ones aren't in production anymore, and it doesn't look like there are any replacements on the horizon. But with that said, they resumed Twin Otter and Do228 production with upgraded avionics a few years back so maybe we'll see that happening with other designs.
