After the past several decades I have answered this question for many clients. I do not believe in upgrading technology without a valid reason. I have used the following criteria when working with clients.
The reasons to change out a phone system:
Change in environment - if you are moving, adding a location, adding people, and that requires an investment in your current phone system you should ask the question is it best to invest in what I have or look to something else. This can also be true if you are downsizing. Paying for outside lines and maintenance on a phone system for the size you "were" may not be prudent.
There are new widgets - sometime there are new products that can show efficiencies that drive people to upgrade their phone system. Implementing a Contact Center, adding features to your call center such as text and email, mobility and integrating your mobile phones into your enterprise system are a examples of "widgets" that an enterprise may be able to benefit from.
The system is losing service and support - Manufacturers are all taking away support of old TDM systems. This is hard to tell management that it is time to invest when the system is doing everything it should. The risk is that a problem may take your business down for days. A software glitch may not be able to fixed because it is not supported, parts may be hard to find and only available via secondary market (like EBAY), and often most important vendors are losing trained staff on these products. Techs trained on the product are promoted or retire and companies do not train the young employees on old systems that do generate little revenue for the company. The questions you must ask are what is my risk? what is my backup plan to restore the system, what is my plan if the system simply goes down and I need to replace is quickly?
Save money - Of course cost is always a consideration. If you have less than 40 telephones and you have a maintenance contract, a PRI from the phone company, and old internet connection you may be able to get a new hosted telephone system and be close to revenue neutral. If you are around 20 phones you are likely to save money and get a new system.
Answer these questions and see if it is time. Contact a trusted advisory if you need help. Remember things in this industry are changing really fast so an annual review is a good idea.