It's the weight of our rigs that's working against us here. Keeping 3 tons(or 4+ in some cases) from crushing the occupants in a crash is tough work for skinny A-posts. And the engineering and metallurgical components of the design, while quite good at the time, are now probably 25 years old. Today there are nickel-boron alloys and other stuff being incorporated into some vehicles that's so tough you can't cut it with the normal emergency rescue tools, it has to be pried away or sawed through. Crazy strong stuff, and boy it really does work.
Thing is, there's two kinds of crashes we're talkng about here, the rolling down a mountain type at relatively low speed with multiple impacts over a few seconds and a high speed highway type of crash- typically with one very large impact over only a few milliseconds and then maybe some smaller ones. The impacts and injuries sustained are very different between these two kinds of crashes, and as much as we love them the protective qualities of the 80 are now outdated, even on the airbag equipped models. (I'd still take my 80 over a Prius if I knew I was going to have a wreck, but that's just my own prejudice at work. Statistically the newer car is safer, I say

that, gimme a big truck.)
Based on 30+ years in emergency services I'd be against an interior cage in pretty much any on-road vehicle unless you went for the 'full meal deal' complete with helmets and five-point harnesses. I've seen too many injuries from striking the interior of vehicles in crashes. There's three impacts in every crash, the first is the vehicle striking something, then the occupant striking the inside of the vehicle, then the internal organs striking the restraints, which may be either the skeletal structure of the body or coming up hard against the seat belt or interior vehicle surface. Air bags spread the force of the impact over a short period of time and over a greater surface area to help lessen the damage. Some kinesthetic models also show a fourth impact where the organs recoil inside the body but this is pretty minor compared to all the other forces involved and usually the only serious injuries from this may be head injuries where the brain gets sloshed around like a sponge in a bucket. (There is a traumatic brain injury pattern called a coup-contrecoup injury from this type of force, where the actual brain damage is on the opposite side from the impact.)
My point is, an interior roll cage will not help at all in a frontal impact, may make things worse in a side impact, and can really only help by keeping the roof from collapsing in a rollover, in which case the vehicle may survive but not the occupants if they smash their skulls on the cage during the accident. Or, they may just be drooling in a wheelchair and wearing a diaper for the rest of their life due to surviving the crash but receiving severe traumatic brain injuries. Could you get these injuries without the cage? Sure. Would you be more
likely to get a head injury if you put strong metal tubes close to your skull? Absolutely. You can't put enough padding on a cage to make it safe in a severe impact unless there is other protective gear(like a helmet) being used, not without having the cage padding be so large in diameter that there's just no room for it.
An exo-cage would be a lot safer for the occupants in any significant wreck. And they look cool as all get-out too, but that's beside the point. Now this is a (mostly) free country and folks can do (mostly) whatever they want, even if it's putting the equivalent of 20 sticks of dynamite(that's 5 gallons of gasoline as fuel/air vapor) on their back bumper where any idiot texting while driving can smash into it. Hey, knock yourself out. If you want an interior roll cage then it's your money- go for it. Just please think about it first and then consider filling out the organ donor portion on your driver's license, because a good percentage of those donated organs come from folks that are brain dead after smashing their skulls in various traumatic accidents. You could really help somebody that would have no chance at life otherwise.
As always, this is only my opinion and your mileage may vary.