The Steam Deck has made handheld PC gaming much more affordable so it should completely dwarf the sales of all previous devices like it. Due to this and future revisions and competitors there will now be more options for handheld gaming that aren't really expensive for those who like or prefer playing games that way which makes me wonder how much the Switch's successor will be impacted by that since the Switch has benefited from a lack of direct competition in that area. For those where the appeal comes from being able to switch from handheld to dock play and vice versa some of these other options will probably try to push that aspect to further increase appeal. 

This rise in handheld PC gaming has come about too late in the Switch's life cycle for the impact on it to be any more than just slight at most but the successor will have these other options impacting it from day 1 so the impact will be higher though I'm not sure by how much exactly. What do you think?

Unless Nintendo starts publishing their games on PC, I don't really see handheld PC's making much of a dent on the Switch's successor. The sales of Nintendo's first-party games on Switch show that's the case. Gamers, casual or hardcore, buy Nintendo hardware to play Nintendo games. 

And with Nintendo teaming up with Denuvo, ugh, it'll make it harder to emulate Switch games at launch on PC. So I think the impact will be minimal at best.

I think the steam deck will impact sales, at least a little bit.
With how expensive everything is getting these days most common customers may only be able to buy 1 system to carry them through a whole generation if not longer.

I was speaking to a friend who was telling me that they got the biggest steam deck model and they had no interest in buying a bigger switch model from Nintendo as it would cost them too much money to own both.

Edit: for me personally, I think it depends mostly on prices, will the next gen switch cost $400+, will Nintendo jump on the $70 game price along with everyone else, will NSO ever be worth the asking price?

Handheld PC gaming has been around for years, didn't impact the Switch, so I doubt it will impact the Switch 2.

Hard to tell tbh. If it's just Valve doing their thing in the west and they only sell Steam Decks through Steam, then there won't be much of an impact even if Steams userbase is large. But if Valve or a different company starts putting them in customer facing stores like Bestbuy/Walmart and etc and they have some of the kinks worked out along with much higher manufacturing capacity. Then maybe a bit more of a dent but I suspect it will be minor regardless.

About as much as desktop PCs will impact PS5 and Xbox Series, or for that matter PS4/Xbox One before them, and PS3/360 before that...

It's hard to say really. If Valve continues to produce them at the rate they are I can't really see it doing much if anything to a Switch succesor. The only way it could make a dent in Switch sales. Is if it got sold in retail stores and Valve majorly increased production. It also needs more marketing.

None

Seems fairly safe to say that Nintendo's next system will be a hybrid, but still not 100% on that.

Either way, don't think it's going to make much of a difference, because the features aren't the same. Assuming you're staying within the bounds of the law, or you're not very tech savvy, Nintendo games are going to be the main selling point. Multiplayer is much more practical on the Switch, particular the rooftop party kind of multiplayer. It's unlikely that a good PC gaming machine will ever get near Switch size, because if it's going to be able to run modern PC games, it's hard to get to a handheld form factor.

Most importantly though I think, console gaming has been fine despite PCs doing pretty much anything a console can do. For whatever reason, a lot of gamers just prefer the simplicity, closed ecosystem, etc. So, I don't see why PC gaming would severely hamper the Switch in particular, when Sony and Microsoft's console business has been fine.

I do expect an uptick in people like your friend in the coming years. Not sure how big of an uptick though, currently it feels to me that the lost sales could be as low as a few hundred thousand or as high as a few million for the successor.

It is becoming more popular now though which is notable. For a lot of people the Steam Deck will be their first time buying a device for that purpose.

I'd say the impact desktop PCs have had and will have on consoles like those is massive since if it wasn't an option to play video games on a PC then the only way for tens of millions of people to play games like GTA would be on a console. If that wasn't an option I think the combined Playstation and Xbox sales would be increasing each generation instead of decreasing a bit each time so far. Now there won't be nearly as many handheld PC's being used for gaming compared to desktop ones though of course so the impact on the future of the Switch line will be way smaller but I think it'll increase overtime.