Going off my region (US), NES to SNES was six years. SNES to N64 was five years. Genesis to Saturn was five years. Saturn to Dreamcast was five years. N64 to Gamecube was five years. PS1 to PS2 was five years. Xbox to Xbox 360 was four years. PS2 to PS3 was six years. Gamecube to Wii was five years.

As you can see, five/six years was pretty standard up until Sony and Microsoft thought it was wise to drag the PS3/360 corpses far past their expiration dates, and the former was the era I grew up in, so it's what I'm use to. Anything longer than five years, and I'm checking out. Heck, at four years things start to get stale for me. I like new consoles, new experiences. It's just my preference.

The PS5 will likely release in 2028 or later, and Xbox will follow Sony’s lead. It might launch a few months or even a year earlier, but no more than that

Releasing an Xbox in 2026 is asking Microsoft to repeat the mistake Sega made with the Dreamcast. Right now, PlayStation is the clear leader in setting the standard for gaming development. This wasn’t the case during the PS4 era (at least not in the first half), but it’s undeniable now. The PS5 is on track to outsell the Series X/S by 2.5x to 3x over its lifetime

No developer is going to waste resources making "next-gen" games for a 2026 Xbox, and Microsoft is poorly suited to support their own hardware. With all games targeting PS5 specs through at least 2030, you’ll be stuck with a $600 machine on a closed platform playing slightly better-looking games than you already can on your PS5

Then, in just two years, Sony will launch a platform with more than twice the horsepower of the next Xbox, capable of running all next-gen games far better, for a similar price. At that point, Xbox sales will plummet, and Microsoft will have no choice but to exit the console market for good

That's because Sony (and Microsoft) decided to make too powerful machines. That creates many problems for developers: the costs are boosted, the development of a single game is insanely long, the teams have to be huge... or simply become "another indie studio" like the other 10 thousand ones, launching games no one knows... everything is worse when you launch a machine with "giganormous" tech specs. You are stressing the developers and even the big companies.

The Gameboy case was an anomally. Very bizarre. That console was over by 1995-1996. What happened was 2 things:

1, Sega greatly abandoned the development for the Game Gear by 1995 and planned no revision of it (like Nintendo did with Game Boy Pocket). So GB was the only portable..., by 1995.
2, Pokémon appeared in Japan in 1996, and boosted the GB sells, so... Nintendo planned to launch the game in US and Europe by 1998 and 1999. By then, GB was old as fuck. Nintendo knew that, and first, planned a new 32bit portable around 1995, to succeed the GB around 1997-1998... but then cancelled it when it was already testing the new machine, and just decided to make a new revision of the 8bit GB... with color screen. Why? to make pure profit with the huge Pokémon-mania around the world what started in that moment. And it worked. GB pocket and GB Color sells were huge by then.

That unexpectedly expanded a LOT the life of that machine. But I can tell you, in 1996-1998, the original GB was clearly dying: Its better and most known games were the very old ones, from 1989 to 1993 (apart from DK Lands) and practically did not have new launches from Nintendo.

A lot of it is specs and long development times. 

If a AAA game took 2-3 years almost all the time, consoles would probably have 5-6 years before they were replaced. Now because of a lot of games taking 4-6 years, the cycles need to be 7-8 years. And even when the AAA games take 3-4 years instead of 5 or more, the teams often have to work on more than one IP. Thus, you need the long cycle to get at least one game out in an IP. 

The Last of Us Part II took about 70 months, or a little under 6 years. Pretty sure Sony didn't want to launch PS5 until a title like that was out. 

Imho, PS6 is a machine for 2028.

They need something that's way ahead of PS5pro at a very inviting price. 

Depends on where technology is at, which is what will determine the next-generation console start date.

If PS6 is going to be sold next to PS5. With PS5 according to Sony cannot be sold cheaper due to manufacturing cost, I'm going to have a hearth attack when Sony reveals the PS6' price.

I don't think it can work like that because they have to plan years in advance. I think Sony is just going to continue to release consoles and at this point they don't care if you upgrade to the latest one. I think everything will be on PS5PS6 from here on out. I don't think we see anyone cut off the PS5 (including Sony) till maybe the end of PS6 if that. They'll be approaching it like PCs pretty much. Upgrade if you want to or don't and stay on PS5 and you'll still get the games. They'll wait/hope PS4 users will eventually come around once their systems break down or the PS5 hits a low enough price for them.

If the technology doesn't exist to make a next-generational console a viable jump, then they won't release a console.

PC GPU technology jumps are slowing and becoming less impactful as time goes on.

You are right, it does take years of planning, but the above also still holds true as AMD tends to be working on next-gen GPU designs years in advance, so they have an idea on performance/capability projections even as early as today.