Should you decide that you are not quite ready to switch full-time to REACTOR, you can of course keep Simplify3D installed and continue to use Simplify3D as your slicing software.
However, please remember, there are known issues that significantly impact the performance of your Fusion3 3D printer caused by versions of Simplify3D. This is one of many factors that contributed to the decision for Fusion3 to develop its own slicer software. Should you continue to use Simplify3D, these issues may continue.
Also Simplify3D 4.0.1 is already over 3 years old (released in July 2017) and will get progressively more and more out of date.
We will continue to encourage all users to upgrade to REACTOR in order for our customer support team to best assist you and will become our standard in the future.