They *could* produce the firmware and updates forever, but how are they going to pay the salaries of the people and infrastructure to deliver it? Yes, they could build it into the price, but if they built in 'everlasting support' into the original purchase price, it would be too expensive for you to buy in the first place. It's not only up to the SonicWalls of the world, but also the chipmakers (and every other supplier up the line).
The folks that front the money (so you don't have to fork out the purchase price 2 years ahead of time) want some return on their investment. If technology was static, the 'perfect' product could be made, it would be flawless the first time out and never need replacing. But technology isn't static, and there is no such thing as infinite patience.