The Lamborghini Sian is an impressive supercar with a hybrid system that uses a supercapacitor instead of a traditional battery to store energy. While this technology offers some advantages, there are several reasons why it's unlikely to become mainstream in the automotive industry.

First and foremost, the cost of supercapacitors is significantly higher than traditional batteries. This makes them an impractical choice for most consumers, who are more interested in affordability and efficiency than the unique features that come with a supercapacitor. Lamborghini's high-end customer base is more willing to pay for such unique features, but for the average car buyer, the cost is likely to be prohibitive.

Secondly, while supercapacitors can provide a high level of power output, they lack the energy density of traditional batteries. This means that they can't store as much energy as a battery of the same size and weight, which limits their practicality in many applications. In the case of the Lamborghini Sian, the supercapacitor is used to provide an instant boost of power during acceleration, but it doesn't have the range or capacity to power the car for extended periods.

Lastly, the technology is relatively new and untested in mainstream applications. While supercapacitors have been used in some niche applications such as buses and trains, they haven't yet been widely adopted in the automotive industry. This means that there is still much to learn about their performance, reliability, and safety over extended periods of use.

In conclusion, the Lamborghini Sian's supercapacitor hybrid system is a fascinating technological innovation that offers some unique advantages. However, it's unlikely to become mainstream in the automotive industry due to its high cost, limited energy density, and untested performance. While the technology may find some niche applications in high-performance supercars and other specialized vehicles, it's unlikely to replace traditional batteries in mainstream consumer vehicles anytime soon.