Looks like we may be seeing a similar double helix pattern to the Switch and PS4 for their first few years.

The double helix will continue next year, because the Switch didn't really do that great in its second year.  Sometime in 2023 though, the double helix will permanently end.  That will correspond to when Switch sales really started to grow.  Meanwhile PS5 will peak in 2022.  It's all downhill from there.

I still think you're massively underestimating the PS5.

It's the fastest selling system ever even with severe supply constraints, and said constraints are apparently set to remain until 2023, so I can't see itthe system peaking while its held back by them, especially when a second year peak is not characteristic of a Playstation home console.

Don't believe the hype.  It is not the fastest selling system ever.  People keep repeating lies, and much of the internet keeps eating it up.  A post in this very thread proves it is not the fastest selling system ever.  Look how the Switch has sold more.  The Switch line is above the PS5 line.
https://gamrconnect.vgchartz.com/post.php?id=9330901

More importantly, the PS5 is an incredibly frontloaded system.  Hardcore gamers eagerly buy it up, but when it comes time for the mass market to buy, they'll go more for Switch and X|S.  I really do expect it to peak in 2022.  In Japan, I expect this year is the peak.

Why on earth would it be so front-loaded when its first few years are severely supply constrained, and in terms of appeal it's very much in line with the PS4 which wasn't front-loaded?

PS5 has a whole lot in common with the N64.  Both are successors to systems that dominated their generation.  However, both are destined to fall.  You have to look at the factors that cause the numbers, and not just the numbers themselves.  Going by the numbers, the N64 was the fastest selling system in its first 1-2 years.  The most enthusiastic console gamers could not wait to buy one.  However, if you looked closely at the time, then you might have noticed CD's were a clearly better choice over cartridges, especially for developers.  An alert person would have seen the tides shifting, but you'd never have seen it if you went purely by the sales numbers for the first 1-2 years.

The main problem with the PS5 is that it's really only going to be popular in Europe.  Japan PS5 sales will fall next year, because no one there is buying PS5 software.  RoW sales behavior is not too different from Japan.  In North America, XBox is arguably a stronger brand than Playstation.  With Microsoft clearly putting in a lot more effort this generation, they are going to coax back a lot of 360 gamers who bought a PS4.  That leaves Europe.  PS5 will do well in Europe.  It's going to lose a lot of ground everywhere else.

Except that PS5 is beating Xbox in the US.

N64's extremely front-loaded sales were due to the exodus of third parties from Nintendo; nothing of the sort is happening with the PS5 that would cause sales to collapse, it's following the very safe and proven formula laid out by PS4.

XBox will be beating PS5 in the US once it starts releasing some good exclusives.

Third parties are actually heading to Switch in Japan.  The very same thing is happening.

PS5 has a whole lot in common with the N64.  Both are successors to systems that dominated their generation.  However, both are destined to fall.  You have to look at the factors that cause the numbers, and not just the numbers themselves.  Going by the numbers, the N64 was the fastest selling system in its first 1-2 years.  The most enthusiastic console gamers could not wait to buy one.  However, if you looked closely at the time, then you might have noticed CD's were a clearly better choice over cartridges, especially for developers.  An alert person would have seen the tides shifting, but you'd never have seen it if you went purely by the sales numbers for the first 1-2 years.

The main problem with the PS5 is that it's really only going to be popular in Europe.  Japan PS5 sales will fall next year, because no one there is buying PS5 software.  RoW sales behavior is not too different from Japan.  In North America, XBox is arguably a stronger brand than Playstation.  With Microsoft clearly putting in a lot more effort this generation, they are going to coax back a lot of 360 gamers who bought a PS4.  That leaves Europe.  PS5 will do well in Europe.  It's going to lose a lot of ground everywhere else.