Nintendo says it doesn’t have plans for any additional Super Mario Run content

The complainers are always louder than the praisers.

People complain about SuperCell and in-app purchases in their games, but it's funny to think about how much money they make. Employees and founders there are among the highest earning people in Finland and pay the most taxes among individuals in Finald, so certainly if people are upset they aren't voting with their wallets.

  Super Mario Run isn’t living up to the expectations for Nintendo and investors
  Many investors in Nintendo are worried about the overall profitability of the game
  Investors point to games like Pokémon Go
  Some hoped that Nintendo was planning additional content to make the game more lucrative

  mixed reviews from players

  it may frustrate some players who had hoped Nintendo was finally betting big on iOS

This. People really seem to struggle to learn that if you massively hype something in advance, a lot of people are going to be disappointed. Even if it's good, but just not as good as they were expecting, they will feel disappointment.

It's been advertised on the front page of the App Store for over a month, which is more promotion then I've ever seen Apple do for a game, but although this game is good, it doesn't feel good enough to be the most promoted game in iOS history.

Worse, if you look at the big spenders, you usually find people with mental health issues, and money launderers using stolen credit cards to level up and then sell accounts.

[source: anecdotes from acquaintances with experience with multiple sides of these issues]

Hardly anyone discovers apps outside the 'featured' and 'top paid' or 'top free' lists on the app store, so any loss in downloads drops you down those top lists, which can cost you a serious loss in potential revenue and is thus unacceptable.

Also if you have a free and paid version you're forcing the user to download your game all over again just to purchase it, which adds friction to the purchase, which anyone in sales knows even a single extra click lowers the odds of people buying something by a lot.

So the only thing that makes sense for most apps is to make your app free and allow people to spend money in the app to either unlock or acquire something in the game.

There are some apps that are better served on the paid tiers, but those are usually apps that have enough visibility (or built-in audience, i.e. Cartoon Network games) and possibly a high enough price on other platforms that people would expect to pay something anyway (i.e. Minecraft Pocket, Square Enix games), or serve a niche audience of people that don't mind paying (i.e. app versions of board games whose prices are set at fraction of the price of buying the physical board game).

    But why build a two-hour game and stop there?

    Wouldn't a 30 level expansion be inexpensive to develop and well
    received by fans?

I don't even understand your support over this business model. ATVI supports this model and is down ~$2 on the year. While NTDOY has avoided it and is up ~9 on the year. KING who heavily practiced this model is no longer trading.

Going out 5year there has been little difference in the per-share profit. So the market really doesn't validate this business model. It is a model, it works, but it isn't wildy successful... The only justification I can see for it is chasing trends.

    Once you've built out the infrastructure, isn't the hard part done,
    and additional art/level assets are comparatively less expensive? 

Why sell 30 levels for $1. When you can sell 5 for $1 every 6 months? One is a far steadier revenue stream.