It's not pure speculation (as in just wild guessing) because we know enough about past cycles to make reasonable assumptions about future ones. Nintendo systems run on the old ~5-year cycle. They peak early (no later than the second full calendar year) and ever since the N64 they have declined quickly after their peaks due to the lack of strong long-term software support. Nintendo consoles do not have life cycles like those of PlayStation systems, especially not those of the PS3 & PS4. This means we should expect the Switch's replacement no later than Nov. 2022. The PS4 & XBO are following a path between that of the PS2 & Xbox and that of the PS3 & 360. This is likely the last year they will have at or around peak sales, and they will almost certainly start irreversibly declining next year. That means we should expect them to be replaced in 2020-21 depending on how quickly they decline. Based on everything we know about why and how consoles sell the way they do, the Switch's successor is likely to come out no later than two years after the PS5 & Xbox 4 are released, which will put it squarely in Gen 9.

So, either A) Nintendo has released two Gen 8 systems, B) Nintendo will release two Gen 9 systems, C) we call the Switch's successor "10th-gen" even though it will live and die entirely during the the 9th-gen PS5's life cycle, or D) Nintendo must be excluded from the conventional console numbering system, perhaps using the Switch's hybrid handheld/console nature as justification (handhelds don't really fit as neatly with home consoles in the generation classification system). Those are really the only plausible options here. Option D is a reasonable compromise as Nintendo is falling out of alignment with PlayStation and Xbox as far as timing of console releases is concerned. Option C is an absurdity and should be dismissed out of hand. Options A and B both admit the possibility of two systems from one console maker being in the same generation. Given that the Switch will peak no later than 2019, thus resulting in it having it be in the decline period of its life cycle when the PS5 is released, it makes more sense to classify the Switch as an 8th-generation system. It will spend its best years being nominal competition for the PS4 & XBO, and so it makes sense to put the Switch in the same generation as the PS4 & XBO. Using competition to define generations makes more sense than "System X came after System Y, and System X is Gen n, so System Y must be Gen n+1" rationalizations that people seem to be using to classify the Switch as Gen 9.

Now, I suppose you could say, "Well, the Switch might not peak until sometime near when the PS5 peaks, so we can still call it 9th-gen" but that would be pure speculation as it makes assumptions about sales trajectories that have absolutely zero basis in fact. There are patterns in the sales data. Consoles sell the way they do for reasons that can be analyzed and quantified. The data can be used to make reasonable assumptions about future generations, even if we can't predict absolute numbers. In all likelihood, any claims that the Switch is a Gen 9 system will look ridiculous in, say, about four years.