First-generation rootkits were just normal programs. Today, rootkits are typically packaged as device drivers. Over the next few years, advanced rootkits may modify or install into the microcode of a processor, or exist primarily in the microchips of a computer. For example, it is not inconceivable that the bitmap for an FPGA (field programmable gate array) could be modified to include a back door.  Of course, this type of rootkit would be crafted for a very specific target. Rootkits that use more generic operating-system services are more likely to be in widespread use.
The kind of rootkit technology that could hide within an FPGA is not suitable for use by a network worm. Hardware-specific attacks don't work well for worms. The network-worm strategy is facilitated by large-scale, homogenous computing. In other words, network worms work best when all the targeted software is the same. In the world of hardware-specific rootkits, there are many small differences that make multiple-target attacks difficult. It is much more likely that hardware-based attacks would be used against a specific target the attacker can analyze in order to craft a rootkit specifically for that target.
As long as software exploits exist, rootkits will use these exploits. They work together naturally. However, even if such exploits were not possible, rootkits would still exist.
In the next few decades or so, the buffer overflow, currently the "king of all software exploits," will be dead and buried. Advances in type-safe languages, compilers, and virtual-machine technologies will render the buffer overflow ineffective, striking a huge blow against those who rely on remote exploitation. This doesn't mean exploits will go away. The new world of exploiting will be based on logic errors in programs rather than on the architecture flaw of buffer overflow.
With or without remote exploitation, however, rootkits will persist. Rootkits can be placed into systems at many stages, from development to delivery. As long as there are people, people will want to spy on other people. This means rootkits will always have a place in our technology. Backdoor programs and technology subversions are timeless!