When the idea of Nintendo Switch started floating around, a benefit many fans hoped, and one Nintendo themselves hinted at a few times, was that 95% of Nintendo's resources can now be under one roof, ensuring that those Wii U era software droughts are a thing of the past. When this thought was first pitched, many fans assumed it simply means all the 3DS games and all the Wii U games will now just be under one roof, plus even more games right....? Not, really.
I've said before that gamers severely overestimate how difficult HD development really is. In reality, games are actually easier to make than ever thanks to modern development tools that allow even the smallest and nichest projects to have the polish and quality they deserve in a much shorter time frame and lower budget than even a few years ago. That being said, they aren't entirely wrong. AAA games are more expensive and difficult to make now and that's due to rising standards. With each generation, games are expected to look better, play better, be bigger, more complex, have better stories, etc. These things add up, and cost increasing amounts of time, money, and resources to make happen.
Nintendo isn't really immune to this. While their games are less demanding than most AAA titles, they still need way more time and resources to make than past generations. A AAA game on the 3DS can be made in at most, 3 years with roughly the size and budget of a mid-tier console production. A AAA game on Switch however, needs at least 80-100 people on it, with more complex environments to be rendered, and can take upwards of 5 years to complete. As such, you're not going to see Super Mario 3D Land and Mario Odyssey back to back within a year of each-other. And it's also not as straight forward as a 2D Zelda and 3D Zelda releasing months apart. So if it's not as simple as 3DS games and Wii U games on the same platform, where are those extra resources going anyway? They go to other types of software.
Exhibit A - You may have noticed that Nintendo's been placing a lot more focus on DLC expansions and content updates for their games lately. That's because the freed up time, allows developers to expand upon their existing games for as long as they want. Why make a handheld equivalent of Splatoon 2 in tandem with it, when you can lift that engine and asset library to make a self-contained single player expansion? Why start work on a new Fire Emblem right away, when you can milk the existing game a little longer with a few DLC expansions in a fraction of the time it takes for a new game? Nintendo's new strategy ensures that even if there isn't a new game per-say, there's a constant stream of new content releasing on a frequent basis.
Exhibit B - Projects like 1-2 Switch, Pokemon Let's Go! Sushi Striker, Wii U ports, and Nintendo Labo. You may not be a fan of them, but these are also replacements for handheld entries of series. Allowing developers to experiment with low-risk, low-budget concepts with a lot of creativity along with cheap ports adds variety to a yearly release calendar, even if they aren't major sellers (or complete bombas in Sushi's case). Nintendo's done these types of games before, but their importance will become more prevalent going into the Switch I believe.
Exhibit C - The extra staff now allows Nintendo to freely assign more people to a project when needed to ensure it meets a deadline. Nintendo want's Metroid Prime 4 to make a 2019 release date? Well they can simply assign support staff to the core development team later in development to get the game out on time. Before, they were always at risk of pulling staff away from supplying enough games for 3DS and vice versa. Here, it's much more free-form, and they still have time to work on other projects too.
So Nintendo's not going to put out more games than they already do, nor does it mean all the handheld games and console games are together at last. But it does mean games can last longer with new content more frequently, AAA games can come out faster if needed, and the more experimental games can take the place when needed.
TL;DR - Unified development for Nintendo on Switch doesn't necessarily mean handheld games and console games are on the same system due to increased time and budgets for AAA games. But it does mean more content for existing games, as well as more experimental types of games and enhanced ports, as well as AAA games getting out faster when needed. All of this ensures a steady stream of content coming to the system at all times.