In terms of first party games, I largely agree. If they launch the drake as a successor as I envision, then most first party games will probably be drake exclusive to get people to upgrade. Nintendo would probably still give it some support, particularly lower budget games and ports/remasters.
But in terms of third party games I think the switch could thrive for a long time, even with a successor. There's going to be like 130m people with a regular switch, and that's a lot of potential software sales.
Also one thing I wonder is if Nintendo might retroactively upgrade Switch's NSO. Let's say they add gamecube and GBA games to the drake, these could also probably be added to the regular switch in lower quality.
And keep in mind when the next console comes we could also see price drops on first party software to increase sales of these existing games. It wouldn't be shocking to see Mario Odyssey eventually on sale for $25 for example. This could help sustain software sales on the previous console despite a new shiny one being out.