Indeed - just because it is *less* safe doesn't mean that it is *unsafe*. There are certain safety upgrades (e.g., airbags and ABS brakes) that can't be done. That's fine.
Others - like 3-point belts and disc brakes - are not only possible, but reasonable. In fact, it would be unreasonable to take that car out on the road without them. It wouldn't only be my life at risk; I'd potentially put others at risk, too. Although I might be at greater risk of getting bruised and dinged up in an accident, with some basic upgrades I'd be at a much lower risk of being crippled or having my teeth knocked out by my steering wheel.
And besides, if safety were my only priority, I'd trade in my '18 Challenger R/T+ for a Camry.