With the Nintendo Switch, you have multiple ways to buy the apps, through multiple vendors, with multiple options for payment. As a gatekeeper, the market is open.
Nintendo Switch games can be purchased physical or digital, using any payment method, online or offline at your discretion. Even if Nintendo gets a cut, it's not part of the PAYMENT, in other words, you're paying $50 for a game no matter where you pay for it. This isn't about licensing fees, this is a quite literal Apple tax with Apple pay. If all app developers had a choice of Apple Pay with a fee, or external processing systems, it should be their choice. There's no "security" in having everything run through Apple. I mean, my mortgage payment doesn't need to run through Apple Pay for "security" ... it's ridiculous that Apple somehow thinks their payment system is better than everyone else, frankly. At the developer's expense, too.
Effectively sideloading for Nintendo is possible as well; the only real limitation I am aware of (like Apple) is: both systems have APIs that provide functionality. That's why jailbroken iPhones can run any application, not because they're special apps, but because it gets beyond that only-one-storefront barrier. Once the app is on the device, it'll run just fine. Not because it's bad, but because like any operating system, with coded APIs, it'll do what it is told.
So is Nintendo engaging in monopolistic practices? Not in the slightest. There is nothing wrong at all with creating a great device that has a specific targeted audience. Nintendo isn't restricting developers and consumers, isn't taking a percentage of every sale everywhere and preventing advertisements and preventing even knowledge about where to acquire product. They endorse it.