I found the different types of installation methods and the pros and cons of each to be fairly interesting. While direct installation is most likely the quickest and least expensive implementation option, it comes with the most risks. And trying to save some time and a few bucks could end up costing you significantly in both areas if something goes wrong. Meanwhile parallel installation means running two systems at the same time and duplicating work. I can imagine that as an employee that would be a frustrating situation to have to, for example, enter information twice. The phased and single-location options are kind of a happy medium. I’m sure it all depends on the size and specifics of the project and organization, but I’m curious what the most popular, go-to installation strategy is.
I would like to add that personally my company currently is spending most of their IT budget on system maintenance actually, so I concur with you on this one. Both phased and direct implementation are expensive options; however, both require costly maintenance programs. Since the phase can begin soon after the system is officially and fully installed, the program can be an expensive endeavor. As with the initial design of a system, maintenance activities are not limited only to software changes or updates, but can also include changes to hardware and business procedures.
It is interesting how these installation approaches have influenced software architecture and SDLC. You could make an argument that DevOps/CICD and an API based software architecture are implementations of continuous small changes that can be done as phased installation. In this way there is almost always a back out strategy (revert back to the prior code) and the s/w (API based) is architected to allow changes to occur in phases. As a result the latest and greatest modes of s/w delivery fit into the phased installation model!