Not guaranteed, but relatively close to a guarantee.
Every Nintendo handheld (minus the bizarro tabletop Virtual Boy) has been a good to earth-shattering success. Nintendo's home console lineup is much more inconsistent. The Nintendo 64 and GameCube sold well below expectations and the Wii U was an outright failure.
A lack of first-party games people are interested in or a high price could keep consumers away from a Switch 2 or 3. Then they are some other factors like marketing/brand image that could tank a platform.
I think Switch 2 sells 60 million in a worst-case scenario. Switch 3 could be an outright failure.
Lifetime Sales Predictions
PS4: 130 mil (was 100 million) Xbox One: 55 mil (was 50 mil) Switch: 110 million (was 73, then 96 million)
3DS: 75.5 mil (was 73, then 77 million)
"Let go your earthly tether, enter the void, empty and become wind." - Guru Laghima