From a bean counting perspective, it makes a lot of sense that future generations of console should last longer and longer. Unless the business model massively changes, there are a number of variable that were true in 1985 are true today and will be true in 2030.
One of these variable is theinstalled base. I doubt any single home console will ever sell more than the PS2, but on a generation basis the market is somewhat expending (currently, PS2 + XBOX > PS3 + X360 but the PS3 will still be sold for a number of years and the Wii opening a market of its own changed the dynamic).
Another of these variable is the sellingprice of games: The price of the standard new game has decreased over the years and the average even more so with the re-release of best sellers at reduced price (something that was introduced for consoles with the PS1 if I remember well).
A third is thedevelopment costs: No magic here, these are on the rise. The more powerful the console, the more detailed the assets, the more time it takes to make them. This is mitigated by know-how and the increase in power of the tools used to create these assets. Something to keep in mind: it took one day to create a car for GT on the PS1, one week on the PS2.
At this point you might see a dream breaking thing: the AAA game, the game that tries to make the most out of the power of the hardware, is constrained by cold hard bean counting to the point where the power of the hardware will become irrelevant, because the cost of making full use of the hardware will be too big to ever hope to turn a benefit (not to mention that from a visual fidelity perspective, there is a brutal diminishing return on power investment).
Let's add a fourth variable, theinitial investment: when you start working on a new console, you need to buy new dev tools, you need to learn to use them, you need to figure out the in and outs of the new hardware and this takes money and time, and time is money.
Consider this fourth variable and the fact the installed base does not go magically from 0 to MAX in a day and you can see that the longer a generation last and the less of an impact this cost has. On top of that, each new game in a generation benefits from the know how, the tools and the engines created from the previous games. With the increased asset creation costs, longer generations give more chance to publishers to actually make money.
That being said, one of the bright mind at the head of Ubi Soft wished for new consoles a couple years ago, to create excitement and energize the market, because it seems consumers simply grow bored of their hardware if it lasts too long. Also, business models do change: DLC, sponsored exclusives, retailer funded exclusive DLC, subscription based games, in game advertisement... these are all tools to make it viable to create high budget games.
