It's hard to use history as an indication for future Switch sales because if you look at history, you saw Nintendo's development being split between 2 systems, and then those 2 systems would each get a successor. Essentially juggling 4 systems at one point (like in 2011/2012). Now with the Switch they only have one system to develop for and that system's eventual successor, cutting it down from 4 to 2 near the end of a generation/start of a new one.
What software do you think they'll have ready for a Switch successor if it releases in 2023? Mario Kart & 2D Mario would be big possibilities, but what else could they have? Most of what they released/will release in 2020, 2021 and 2022 probably won't be ready for 2023, 2024 and maybe even 2025. It just feels like they'd be rushing if they release a successor in 2023.
Also, I don't think it make sense when Nintendo keeps talking about extending Switch's life. Feels like Switch should have the longest lifespan for a Nintendo system (from launch to successor's launch) and since PlayStation & Xbox have had 7 years per generation, I think Nintendo should do at least that much in order to keep up.