B
Yes, it is important to fix vulnerabilities inside a corporate environment, even if these hosts are not internet-facing.
Why? Because if the endpoint is used by an employee, it can be compromised via phishing, malware, breached VPN access or similar means. This means your idea of a "safe environment" doesn't hold up to reality. If your security is based on the fact that you don't expect an attacker to be able to access the application, then one compromised host can lead to compromise of many or all hosts running the vulnerable application.
This leads to the next point, which is Lateral Movement. Lateral Movement is the process of an attacker compromising other hosts with similar privileges, in order to maintain their access and get access to more information.
For example, having access to every employee endpoint could enable an attacker to perform more sophisticated attacks, such as stealing tokens from the local system administrator, or finding credentials inside a maintainance script for the Domain Controller.
As a final step, an attacker can then use the gathered information to attack high-value targets, such as Domain Controllers, Database Servers, etc...
To Summarize: While the vulnerable native application was not the first step in this hypothetical attacker's path, it was instrumental to giving the attacker access to a wide variety of hosts, which in turn contained a high amount of usable information.
As a result, it is highly advisable to remediate vulnerabilities in native applications, even if they are only run inside a corprorate environment.