It is easy to see the appeal of devices that have different features to offer... But in order for one device to really replace another, this assumes that one is basically a subset of the other. For example, let's compare Android to itself. There are few reasons to buy an Android tablet when you already have an Android phone. Namely, the larger screen and a keyboard accessory if you have one. Yet people still buy tablets because...? I presume, because it's more convenient than a laptop and the bigger screen is worth it for them. In order for this to be viable, it has to be a decent alternative to a laptop for a number of tasks. At the same time, it is impractical for most people to consider getting just a tablet and not owning a smartphone. Despite the tablet being "bigger and better", a phone wins in the categories of portability and it's an actual phone that you can hold up to your ear and make calls on. The differences between a tablet and a phone are enough to justify different devices concurrently on the market.
The question is, how closely will the Switch emulate the tablet model? Probably not enough for it to be a replacement. Being a gaming device, the Switch will focus on gaming first, and anything else is just icing on the cake. At first glance, you might think the Switch will have a big screen, games, and apps, thus it is like a tablet. However, what sets Android and iOS devices apart is specifically the OS. The OS is specialized around the idea of developers creating apps which extend functionality. Unless the Switch OS provides enough variety AND accessibility in its development model, it is highly unlikely to achieve the same level of app development that tablets receive. Thus, the lack of variety in apps means it is very unlikely to serve as a laptop-alternative, which is one of the main selling points for tablets. The Switch will not replace traditional tablets for most users.
The place where it makes a difference is that it is NOT an Android device and IS a gaming console. That puts it in a separate market altogether. Developers who are interested in serious game development will be interested in developing on Switch, whereas the casuals that fill up the mobile market will largely stay in their own square where they are welcome and comfortable. To frame it another way: While tablet-like productivity apps may benefit Switch users, there is practically no benefit for Android developers to put their efforts into such a platform; they already have a platform that gives them everything they need, that is much more accessible than Switch is likely to be.