Different kinds of needs
When you create a new product you may try to satisfy a need your customers have. Needs for products come in different varieties. Some products you can't live without after you've tried the product once, they become a basic necessity, like electricity. Other products you only need once in a while, in a specific situation, or for a particular kind of work, like a hammer.
Software products also seek to satisfy different kinds of needs, and software is for most people a basic necessity required to do work. We use software for general purposes such as communication in text, voice and video, and we use software for very specialised purposes such as designing satellite antennas.
Still, the number of apps that are required by most people to do their jobs are very limited, and creating something new that becomes a basic necessity for doing work seems exceedingly difficult. Many have tried, few have succeeded.
Why did Microsoft's office suite become the ubiquitous software, that is now a requirement to run a business? Why is Slack now growing to dominate all company internal messaging?
The answers to both questions are complex, but involves releasing products at the right time to the right audience. The process that lead to wild success cannot be replicated.
The market for general purpose business software is vast, but dominated by a small number of vendors. The same is true for the very specialised software (think satellite antenna design). But, if you have sufficient domain knowledge, you may very well be able to create a special software that will dominate a tiny niche. The problem is that a niche is small, and your software may be difficult to adapt to other niches. Growing your business may be difficult.
The market for slightly specialised business software is also gigantic, and is less dominated by single vendors, but has room for many players. The market sustain thousands of bug trackers, shared to-do list, project management and CRM apps. Some more popular than others.
The takeaway is that you are more likely to create a successful product by fulfilling a less common need, than trying to replace email.