Please identify the 'system' you are referring to : as per Mitel's documentation any unpatched Physical or Virtual controller running MiVB 5.0 to 9.0 SP2 software
There are two questions that are very important.
One:
Does Mitel force owners to upgrade for fees a system without any change? No Mitel has provided a patch for free for 5.0 - 8.0sp3 systems, if you have 9.0 9.0 SP2 you will have to have a software upgrade to 9.0sp3 this again is free, it's up to your agreement with your vendor if the engineering time required to complete these task comes at a cost, that being said any good vendor should be encouraging you to upgrade if you are on the lower end of the software scale, simple tasks such as backing up a system has become very difficult without compromising the client system by having to old java and tls versions.
Two:
The system in your statement is type and software load? MiVB 7.2 virtual
Was this tested on systems that are not set to communicate with AMC? Yes it was isolated from the internet
The presumption is that ICP type systems do not have regular communication with AMC ergo license verification does not occur ? There is a misconception that the certificate update is to maintain communication to the Mitel AMC licensing servers, this is wrong the cert is question is used to verify the authenticity of the license already on the controller received from the AMC, when the cert is updated the software version is slightly modified appended by a .9 and on next connection to the AMC a new compatible license file is downloaded.
It is also questionable if the local certificate can be re-signed in the ICP system without AMC connection the way it used to be ? The offline method has long gone, you can license a system via the software installer tool if the tool is installed on the windows pc with internet connection but I haven't tested doing this on a patched system, nor will I try not worth the effort IMO.
If its not broke tweak it..