The problem would be someone has to pay for those chips, the manufacturing lines, etc. etc. etc. and that someone would be Nintendo. 

So yes, they could just sit on millions of mass produced chips for a year for example, but they would also be earning no money on those chips and the sunk cost would likely be high. You're talking about a chip that probably is not going to be that cheap to begin with either, so sitting on millions of them unsold would almost without doubt show up on their yearly financial reports as a massive fiscal year loss that they would also have to explain. No business likes sitting on millions of dollars of unsold inventory. 

I don't really see that as a particularly plausible avenue for them. There can be other practical problems there too like they may not be able to say in year 2 "hey we have a ton of chips we hoarded so lets stop/slow production in year 2 since we don't need that much excess chipsets", the problem with that is Nvidia and TSMC (or Samsung) likely have it written that they get royalty fees and a certain number of guaranteed chips every year ... things like that. Which is normal, the chip designer and the manufacturer need to protect their interests too, it's not just about Nintendo being happy. 

There's also a lot of other problems with stock piling chips a year+ in advance. You would probably be stuck with a constant pile of inventory made a year before it has to be sold which leads to you not being able to get the best price on the chip because you're in a cycle where you're always selling from inventory that was mass produced a year in the past. There would also be problems if you wanted to do a die shrink but you also have a fat pile of unsold inventory on the previous worse node.

There's just a lot of headaches with this route that I don't think they would want to be put in that position. Or I'll put it another way ... the upside on doing this would have be incredibly enormous, like having a system that's selling maybe 10-12 million units by its 8th year isn't really worth the trouble of doing all this. There would have to be something incredible happening with the ecosystem like the system showing no decline or even increasing in sales somehow or something crazy like that. 

The other thing with the COVID pandemic issue is if the point is that the pandemic basically inflated Switch sales, fair enough, but the flipside to that point then basically has to be that with the pandemic restrictions now basically over every where that decline would likely be worse going forward with the pandemic gone. And Nintendo actually does kind of acknowledge this in their last investor QA they cite one reason for the system's decline as being the Switch is having difficulty being seen as a go to entertainment device now that the pandemic is over (basically to that effect) people have more entertainment options. But that doesn't really bode well then for keeping an aging system on the market longer.

Dropping a system earlier just for marginally falling profits may not always be the best thing, the Switch successor could easily be less profitable than sticking with current hardware if you release the successor too early. Nintendo dropped the Wii & DS pretty early in favor of the Wii U & 3DS which of course led to their first loss in over 30 years.

Nintendo wouldn't want to risk losing easy great, more-certain profit with their current hardware in order to drop the successor which is way more uncertain if it can produce profits as high, the Switch still this fiscal year would produce 3.55B in profit.

Nintendo also isn't in any sort of rush like the previous generation to release the Switch successor, they have no competitors who could steal their marketshare unlike with the DS & Wii where the DS had to compete with the upcoming PS Vita as well as smartphone games which were forecasted to kill the handheld gaming market. With the Wii, Sony & Microsoft already stole one of the main appeals the Wii had with the Kinect & PS Move which is part of the reason why Nintendo felt forced to drop the Wii earlier despite being a massive success. Switch isn't in any of those scenarios.

Sony would still release the PS5 despite the PS4 being profitable because Xbox is still a major competitor to them and they can't afford giving MS a headstart in the generation, nothing is threatening the Switch right now for Nintendo to be in a rush.

I'm not doubting , but...

How reliable is that leaker?

Wii and DS had pretty long product cycles, there's nothing short about 6 years on the market. It's a normal product cycle, DS just over 6 years actually and seven actual holiday shopping seasons (04, 05, 06, 07, 08, 09, 10) without a successor. 

But even if Nintendo moved 3DS to 2012 and Wii U to 2013 ... then what? 3DS and Wii U would've done great? Wii and DS would've declined either way, like DS for example was reaching saturation points in the Japanese and Euro markets by 2010 already. 

They lost money on Wii U and 3DS because they were badly managed launches that were banking heavily on trends that were 3+ years past their prime ... ie: Nintendogs 2 would effectively be such a big hit that the it could carry a 3DS launch, NSMB was a big hit on Wii and DS so naturally it could carry the Wii U launch. They were wrong on both of those bets. They had oversaturated the demand for those concepts and people just weren't as excited about it a few years later. Who here was over the moon hyped about getting a 3DS for Nintendogs + cats? *crickets*

The 3DS honestly probably would have done worse if they launched later which would have cost them money that way too. I don't think it hits 75 million+ if they just held it off the market an entire extra year and then tried to launch it in 2012 instead. You're maybe talking 60-65 million units instead. They botched the launch window but by the fall of its first year they were able to get sales at least to a decent level with Mario Kart 7 and Mario 3D Land so losing that first year would have still hurt them. Like the highest selling holiday sales the 3DS had was holiday 2011 ... if they missed that window it would have been worse, not better for Nintendo long term.