Hi Thayanithi,
I think you have some misconceptions here that need some clarification. Konica Minolta is not the only one who is regularly changing their models. Xerox, for example has released 3 upgrades to their Versant line in the last 4 years. Canon just released a new C850 series. Ricoh just release new C7200 series, and so on. It's pretty standard for all of them to release a new model every 2-3 years or so. And this does not mean the previous models weren't stable or good quality, it usually has to do with new technologies such as handling longer sheets, thicker material, new finishers, higher speeds, higher resolutions, and so on.
As for the Konica Minolta naming convention, they used to call off the their machines by Bizhub at the begining and then would have a suffix like Bizhub Pro, Bizhub Press. About 2-3 years ago, they just changed the name of the press series to AccurioPress. The bizhub office series was not replaced by the Accurio line, they are just different classes of digital copiers. The bizhub naming convention now only refers to the office copier line, while the Accuio naming convention refers only to the production line.
As for supporting the current model(s), manufacturers are required to provide parts and service for any model for a minimum of 7 years from the last year it was sold. So if they are not coming out with a new unit for 2-3 years, they will be supporting the current model for 9-10 years. Most will exceed that, especially if that was a particularly popular unit.
The ROI is up to your own business model. I've seen many customers have an ROI after only 2-3 years. There is no set standard for this. Also, most print shops do not purchase the equipment. Instead, they least it for 5 years so they can upgrade to the newest technology at the end of their lease.
As for how many years the machine will print with good quality, my print shop has a Konica Minolta C8000 that is 5.5 years old and it's still printing beautifully. When I used to sell gear, I'd see shops that still had Konica Mionlta C6500's and C6501's that were over 10 years old printing with great quality. I found the same to be true for older Canon, Ricoh, and Xerox machines as well. They are very much like cars that if you just replace the main components such as the imaging unit, fusers, drums, etc, it will keep looking great.