And what's your reasoning as to why you think so?

March 1st or March 8th, 2024, for three reasons.
1) The tech should have progressed enough to a point where we can get a handheld/portable system with PS4/Xbox One level hardware with an acceptable battery life.
2) The 3DS managed to get a 6 year life cycle and the Switch is on pace to far exceed the 3DS, so anything less than 6 full years for the Switch would be a severe disservice. So, it 2023 should be the earliest it launches, but I feel the Switch should naturally go longer than that with how well it is selling and I am confident it's legs will fair better than the 3DS' legs to justify the extra year.
3) It would be closer to the launches of the PS4/Next Gen Xbox than the Switch was to the launches of the PS4/Xbox One. Those systems launched in November 2013, the Switch launched in March 2017, that's 3 years and 4 months between them. If the Switch 2 were to launch in March 2024 (what I'm expecting), and the other two systems launch in November 2020 (What I also expect), that would also be 3 years and 4 months between them.

I'm still thinking it will be holiday season this year, or shortly after in 2020. While it is true that Nintendo has said the Switch successor is not the focus, unprompted, what that did is begin the discussion about the Switch successor; and "we have nothing to say on the matter of X" has always been videogame publisher code for "Stay tuned for details on X" - especially when the President of the company brings it up unprompted. Nintendo was also saying NX wasn't the focus in the months before the reveal trailer, Nvidea was saying that they had no projects relating to Nintendo between the Eurogamer leak and the Nintendo reveal.

I can't see why Nintendo would create a scalable hybrid console and forgo one of the strongest benefits and opportunities of the architecture; iterative hardware upgrades. Especially given some of Switch's first party software would greatly benefit from running on higher clocked hardware. Additionally, Switch software is developed with scalable performance in mind; and as software on mobile phones and PC, usually, it is only an optimization patch that is necessary to take advantage of the upgrades.

I'll go with 2023, I think Nintendo will give Switch 5 years (2017-2022) to itself before launching a new console in 2023.

Nintendo did mention Nintendo Switch having a longer than usual lifecycle. A usual lifecycle for them would be launching a sucessor in 2023. So, by that reasoning it would be 2024 or later.

They also seem to indicate that R&D for a successor is also going to take longer than usual, so I voted 2025.

There will be a ton of Nintendo Switch revisions along the way I am sure, but a full successor seems a long ways off right now.

That's not really a "successor". You aren't going to see Nintendo releasing multiple upgrades in this Switch generation. It may have something equivalent to a New 3DS or an Xbox One X, that isn't a successor. We're not going to see the whole S6, S7, S8 phone esc scheme until probably even after next gen.

2023 or 2024, which also depends on whether they launch again in March or go back to November.

I do expect the main chip this time to be made by AMD. Ryzen will come to smaller form factors (the fact that they extended Stoney Ridge into this year shows that they are working on it currently) and NVidia has repeatedly said they don't want to make custom chips for singular clients, which leaves the X2 as the last workable NVidia option in a hybrid setting from them right now.By then a 4C/8T Ryzen with 6-12 CU (depending on clock speed) and HBM2 memory (goodbye bandwidth bottleneck) should be very possible at reasonable prices under the 5W TDP necessary for the handheld part.

I think 2022 at earliest, that would be 5 years and while I know Nintendo said they want the Switch life cycle to be longer than normal that fully depends on what revisions come along the way, such as the rumored Switch Mini and Switch Pro. 

The problem is once the next gen PS and Xbox arrive in what a lot of people assume will be 2020, that will make third party support infinitely harder for the Switch to maintain as the gap in tech will be even wider. Usually for the first 1-2 years of a new generation games are cross-gen though, so I think any cross gen games (AKA still releasing on PS4 and Xbox One) in 2020 and 2021 can still possibly get Switch ports, but once everything transitions fully to only the next gen PS/Xbox then the gap might just be too wide for most big games. 

I personally don't want to see a Switch Pro as that would just divide the current install base if games come out that can only be played on that more powerful model, but then again that did happen with the New 3DS and Xenoblade Chronicles 3D so it's not out of the question and it probably would ensure they can keep third party support for a bit longer before moving onto their next console. I'd rather them just wait a few years and release a full blown next gen Switch though. 

Nintendo lately prefer the 6y cycle for their successful products (Wii, DS, 3DS), so I expect it at the end of Q1 2023.

Either in 2022, or they'll wait until the end of 9th gen.