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If your late-2013 iMac is running well with a replaced hard drive, you can continue using it for as long as it functions properly. However, since Apple is no longer supporting the device, you will not be able to upgrade the operating system to the latest version, and Microsoft Office will also eventually stop supporting Catalina.
If you need to upgrade the operating system and Office for work, you may want to consider replacing your iMac. A newer model will be able to run the latest operating system and have better hardware to handle demanding applications, such as Office.
If you're not in a rush to replace your iMac, you can wait until it experiences a hardware issue or becomes unable to perform the tasks you need for work. However, it's always a good idea to have a backup plan in case of a hardware failure or if you need to upgrade to newer software.
Ultimately, the decision to replace or continue using your iMac depends on your specific needs and budget.