If the existing system does fifteen things, write down the major tasks of that system and divide them into as many verticals as necessary to describe the broad functional areas that require attention. If the replacement is intended to be a custom-developed, on-premises system, it’s perfectly reasonable for that system to be expected to accomplish most of the fifteen things in question.
However, if SaaS-based solutions are being considered as replacements, keep in mind that SaaS-based solutions are narrow and deep: they are usually highly specialized to take on a very specific vertical and are designed to integrate tightly with other highly-specialized software, feeding cross-referenced information between each other. SaaS solutions are designed to satisfy all your needs in a specific vertical, and that vertical cannot be “the entire operation of the company.”
A good analogy for SaaS systems can be found in your phone. Ten to fifteen years ago, the phone company controlled everything you could do on your phone. Ringtones, backgrounds, music, games, photos, movies, backups, contact managers, themes, apps, and yes even phone calls were provided exclusively through the phone company and there were no alternatives. Today, the phone company “controls” very little. They put some random apps on your phone when you buy it, and then you can go out and get the real services elsewhere. Using just the phone in my pocket, I can access paid, specialized services that deliver their one offering far better than a generic “give them everything” phone company system ever could. I have Netflix for video, Pandora for audio, NHL Game Center for hockey, Google Drive for productivity, IMO for instant messaging, and Skype for video calls. That’s only 10% of the apps installed, and none of them are a whole-life solution. Only one of them, Skype, even does more than one thing. Online software is designed to be very, very good at one thing, and it assumes that the user will acquire more software if they wish to perform another task.
What works perfectly on your phone can also work perfectly for your business, as long as you remember that no single application can stream movies, record video, IM your friends, email your mother, tell you the score of the Cubs game, let you know that your friends are having a beer down the street, and let you listen to the radio. Likewise, unless you have ten million dollars and three years to spare, it’s unreasonable to expect any one solution to do CPQ, contract negotiations, order processing, billing, digital fulfillment, physical fulfillment, printing, mailing, collections, revenue recognition, etc.
Many SaaS vendors have expanded into neighboring verticals. For instance, Aria does billing, dunning, and revenue recognition. If your business has physical fulfillment needs, Aria is more than happy to integrate with the physical fulfillment management vendor of your choice. Just like a link in a gmail message can open a Netflix episode on my phone, so to can an order in Aria initiate a fulfillment management workflow in your fulfillment system. The only thing that changes is the scale, and the perspective of the buyer.