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Once the tycoon/campaign is the priority, how about having retro crazes in the timeline? Such as the late 90s/early 2000s that saw a lot of 40s/50s style cars (VW Beetle, Chrysler PT Cruiser, Chevrolet SSR, Ford Thunderbird, etc), so having a few years where a past 5 or 10 year period is suddenly popular for its designs, so any body in that time period does not have a safety or desirability penalty, and even has a boost in desirability for as long as the retro craze is in effect. Whenever that ends, the models won't lose any safety value, but the desirability boost would be removed. I realise the whole desirability and safety penalty for old bodies is far from final, but when that becomes a priority, maybe consider this.

Safety is more than just body shape, it's crumple zones, pillar size, body bracing,safety cages, and a host of other under skin aspects, that's why there are safety penalties on older bodies they take into account the types of measures that where built in when the body was first introduced. You can pack all the airbags, pretensioning seat belts, and safety glass you want into a 1960's beetle, it's not going to pass a small overlap collision test.

That's what I mean about a retro craze: that it treats the old bodies as new. Basically that they are new bodies but look decades old, so they would have all the safety features of any car of the period it's being built in, not the period the design comes from.

In essence, what you are describing is aesthetics. The retro craze was mostly about looks and nostalgia. Folk thought retro styled cars were cool.
Since it's not possible to programmatically code what looks good, without penalising a players creativity for not meeting that criteria, the Devs have left looks out of the equations.

It has been stated in the past there will be some modern retro styled bodies available once the game is complete and it has also been said that the current old age mechanics will be reviewed as the game is developed.

Plus we have a great modding community, pretty sure you or one of your cohorts will produce something too.

In that case, retro-styled bodies would make sense. Maybe even .lua modding to to make an old body "new" again. Maybe I could make a retro car as one of my future projects

Retro cars from the 2000's still had several design clues (wraparound bumpers, a design that reflected modern aerodynamic trends, modern safety, and so on) that showed that they were still modern cars. The best solution would be to make a couple of retro cars instead.