Engineering is always done to a budget and includes safety factors, plus end products often have to suit a potentially diverse target demographic to maximise sales.
Modding is all about tailoring the car to your needs and making changes the manufacturer deemed too expensive to be worthwhile or didn't have the time/budget to develop and perhaps reducing those safety factors based on the owners desired level of risk. I find it hard to believe changing to poly bushes will have a catastrophic impact on chassis component life, I'd bank on rust killing something first. If making the ride harsher was catastrophic you probably wouldn't get 18" wheel upgrade options with runflat tyres on a car that comes on 16's and large profile as standard.