To a large extent, the skills required to develop an app have become commoditized over the past few years. Also available, are a large number of templates to kickstart app development (specially games). That being said there are some key issues which differentiate successful apps:

User Acquisition:
- Organic
- Paid (at a price that gives best ROI).

Sustained Development:
- Ability of the developer to consistently make changes based on user feedback. I think that app development really starts after the first version is released to the market.

Customer Support:
- How well can the developer interact with customer and build a community.

These are my immediate thoughts.

Protection of your success is paramount. The moment you stick out your head a bigger, fiercer, more experienced, deep pocket players comes around and clones you. Right? Unfair!

Your question is a good one. In my experience there are several things that make it hard to duplicate success even if the app itself is easy to clone. Four come to my mind immediately. On different abstraction levels and certainly interconnected.

1. Protection by IP
Intellectual property. Hard to get. But pretty solid. Often not applicable. Beware of lawsuits against big guns.

2. Protection by user
In essence a speed advantage. Consider switching costs. https://www.investopedia.com/terms/s/switchingcosts.asp
Luring users away from the original can be freakishly expensive

3. Protection by talent
I would always bet on the better team. Often companies get acquired just for talent. A well oiled machine can out-innovate, out-speed, outwit even the biggest opponents.

4. Protection by insight
Markets need to be played constantly. Adjust. Be nimble. Just copying one already working angle does not mean that it will work for long. A team with insight into the dynamics of the apps surrounding can react properly and over time even intuitively.

5. Protection by feedback
Not a very clear distinction to the other points here - its more a practical example of speed (2) and insight (4) - but i want to elevate one point. If you get ahead on the user base you can collect feedback better (quality and quantity) which allows you to react more precisely - giving you the edge.

A really good question!
I've lanched an App and "it has been cloned".
What makes and app successful depends on many things...
- If you are the first launching it.
- Your marketing strategy to reach the correct people
- How the client benefits of your app, is it relevant? Useful? interesting?
- Is your app user friendly and easy to share?
- And many more...

Also there are a Lot of things that the competition cannot copy, and in those things is what you have to focus.

I hope this help at least a Little.

I agree with the observations of the previous answer. Additionally, market strategy determines the app's sustainability. Market strategy enables the app to build Market share, revenue and become viable to acquisition.
1. If an app targets a specific set of pain points
2. Provides easy use
3. Provides a satisfactory user experience
4. Has a unique strategy for acquiring paid users
5. Can grow market share - has global accessibility
6. Is suitable for acquisition
Leading early stage companies to profitability and prepping them for acquisition is what I do.

This is a brilliant question. It highlights that start-ups should not simply focus on launching an App but rather create and launch a brand that happens to exist as an App. Brands are intangible and by their very nature hard to replicate and copy. However, they are also incredibly valuable to companies when mastered in the right way. Would you use another shopping app than Amazon? Someone could clone Facebook, but is it worth it? Brands attract users and can help an app scale and become famous quickly. Also, by building a brand scope is created to live beyond the App format. So, I'd say companies must focus not just on their App, but also their brand. Hope this is useful?