That's a good point. And yes, I still believe it would be better for Sony to have two consoles instead of only one. The amount of sales they got on the 7 gen era wasn't good, but if you look at their PS2 + PSP sales you'll see they sold much more than they could ever hope to achieve with only the PS4 or PS5.
PS2 + PSP combined: 158 millions + 80 millions = 238 millions.
I'm not talking about releasing another portable necessarily, but look at Xbox Series S and X. Two consoles with different target publics in mind (people that just wanna play the latest games vs hardcore gamers). I don't know if Microsoft will outsell Playstation this generation, but I think they'll do better now than they would do by releasing only Series X.
Now answering your question directly, I think Sony and Nintendo are in very different positions. Sony is more than just Playstation, so they already have other revenue streams they could still rely on to make profit, like their TV's. So my answer is yes, even if I believe it could be better for them to make 2 consoles, their single console strategy is sustainable in the long run. Nintendo on the other hand is mainly focused on gaming, if they don't do well with it what else could they do to make money? I don't know if parks and licensing their characters for movies would be enough for them or their stockholders.
Some people brought up Switch and Switch Lite. Well, if Nintendo manages to market them well enough for their target public then I think it could be enough for them (it's basically like Microsoft with Series X and S), however I still think they could sell more (or at least be safer in case their future consoles fail) if they made two completely separate products like before.