I can guarantee you no one wants to go back to that. Not Sony, not MS, and certainly not any of the publishers.
This generation proved that they can run a much wider cycle and consumers will keep buying games. That greatly improves profit margins for all parties. There is no way after proving that consumers will take a 7-8 year cycle that they'll all opt to go back to a 5 year cycle.
The only way we get a new console in 2018 is 1. Nintendo bringing something out because the Wii U is dead and they need to give it another roll of the dice or 2. someone establishes a PS2 level of dominance.
In the second case I'd even bet that it would take MS establishing that level of dominance, leading to Sony (or at that point, their parent company Samsung) pushing out a new console.
If Sony rolls another PS1/PS2 generation on MS I'd seriously question MS's willingness to make hardware for another generation. Instead I'd bet on them looking for an alternative angle into the industry, probably revisiting their interest in Nintendo from a decade ago. A partnership between Nintendo and MS where the former builds the hardware and the later handles the OS, online, etc. would be a very strong competitor in this industry that meets both company's long term needs.