I do not think we should define a tablet as a screen without a keyboard, instead a tablet ought to be defined by how it is most commonly used, by which I mean you mostly use it in portrait orientation like one would a notepad of paper or a book.
In my opinion, a tablet has at least a 7-inch screen and is comfortable to hold and use for long periods of time with the device in a portriat orientation. Consequently devices that claim to be a tablet that have a 12-inch or larger screen are not really tablets, they really fall into a tweener (with notebooks), 2-in-1 category.
Based on how I use my iPad Pro 10.5, it is more a 2-in-1 because it is mostly used in landscape or when in portraint lying on a desk. By my defintion the only true tablets that I own are the iPad Mini 4 and the Nexus 9. The iPad and it’s 9.7-inch screen is probably the largest screen size for a tablet.
My tablet use case definition might account for why tablets have not really overwhelmed the market. Apple had great initial success with the iPad, but sales started to taper off until the introduced the Pro line and significantly decreased the price of the regular iPad. The Pro line really fills the 2-in-1 market, while the lower price iPad has gained sales from holdouts who have wanted one the past but found them too expensive.
Further, by my defintion, the most successful tablet maker is likely Amazon with their cheap 7-inch and 8-inch tablets.