When the number of users exceeds the network's limit, the factor that gets hugely impacted is the longevity of the network.

Longevity:
- The longevity of a network refers to its ability to withstand and support a certain number of users over a prolonged period of time.
- Networks have a limit to the number of users they can effectively accommodate, and when this limit is exceeded, the network may become unstable and unreliable.

Reliability:
- Reliability refers to the ability of a network to provide consistent and stable performance. While the reliability of the network may be affected when the number of users exceeds its limit, it is not the most impacted factor in this scenario.

Performance:
- Performance refers to the speed and efficiency of a network in delivering data and services. When the number of users exceeds the network's limit, the performance can be significantly affected as the network may become congested, leading to slower data transfer rates and increased latency. However, performance alone is not the correct answer to the question.

Security:
- Security refers to the protection of data and resources from unauthorized access, attacks, and breaches. While an overloaded network may be more susceptible to security vulnerabilities, it is not the most impacted factor in this situation.

Therefore, the correct answer is option 'D' - Longevity. When the number of users exceeds the network's limit, the network may experience a significant decline in its ability to sustain and support the increased user load over an extended period of time. This can lead to reduced network stability and reliability, potentially causing frequent disruptions and outages. Ultimately, the network's longevity is compromised as it struggles to handle the excessive user traffic beyond its intended capacity.