Why some Information Technologies remain relevant after many decades?
If you ever had the chance to work with or close to mainframes, you wonder how is it possible that after many decades this technology is still being used and relevant in the majority of business transactions.
On one hand, the simple question is which software technologies stand the test of time; meaning they are still enabling/supporting businesses to basically make money. On the other, the more complex and interesting question is why.
Without getting into the endless debate of which ones are or should be in the list. Let’s review some, extract the essence and understand why. This article was inspired on the GOTO 2021 Online conference .
IBM mainframes are large computer systems produced by IBM since 1952. During the 1960s and 1970s, IBM dominated the large computer market. 
Today in 2022: 67 of the Fortune 100; 45 of the top 50 Banks; 8 of the top 10 Insurers; 8 of the top 10 Telcos; 7 of the top 10 Retailers; 4 of the top 5 Airlines; still use mainframes. 
Why are mainframes still relevant?
They are relevant after 70 years because there is plenty of business logic built in mainframes that is expensive to migrate, because they can still deliver the required throughput to handle large volume of transactions; and because they are reliable.
Structured Query Language (SQL)
Structured Query Language (SQL) is a domain-specific language used in programming and designed for managing data held in a relational database management system (RDBMS), or for stream processing in a relational data stream management system (RDSMS). It is particularly useful in handling structured data, i.e. data incorporating relations among entities and variables. It first appeared in 1974. 
Today in 2022: I was not able to find the statistics, however, companies using SQL must be in the high 99 percent in one way or another.
Why is SQL still relevant?
A key characteristic that I love about SQL is being a declarative language. You tell the database what you want, it resolves how to do it. Interestingly, not all declarative languages stand the test of time, e.g. for instance XSLT suffered a lot due to XML. Another reason is that SQL was designed specifically for data, and it does it really well. What is more, it is simple to understand.
API (Application Programming Interfaces)
An application programming interface (API) is a connection between computers or between computer programs. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how to build or use such a connection or interface is called an API specification. A computer system that meets this standard is said to implement or expose an API. The term API may refer either to the specification or to the implementation. 
APIs originated in the 1940s, though the term did not emerge until the 1960s and 1970s. Later the term expanded from computer to software interface into software to software and now to the web. Nowadays, the web API is the most common meaning of the term API.
Why are APIs still relevant?
APIs as a concept of enabling a connection between computers or computer programs is more relevant than ever. In 2022, APIs present amazing business opportunities to integrate businesses, and they are discussed at high levels of the organisations as a concept to enable businesses.
A successor to the programming language B, C was originally developed at Bell Labs by Dennis Ritchie between 1972 and 1973 to construct utilities running on Unix. During the 1980s, C gradually gained popularity. It has become one of the most widely used programming languages. 
Why is C still relevant?
Since 2000, C has consistently ranked among the top two languages in the TIOBE index, a measure of the popularity of programming languages.
C powers some of the most relevant operating systems such as Windows, Linux; kernels for iOS, Android and iPhone; databases like Oracle, MySql, Postgres; and embedded systems such as sensors, controllers, card readers, etc are likely to be programmed in C.
Key features that make C unbeatable are portability and efficiency, memory manipulation, code size and deterministic usage of resources.
Email is a ubiquitous way of exchanging messages, so popular that it almost seems it has been around forever. Email’s earliest development began in the 1960s, but at first users could send e-mail only to other users of the same computer. In 1971 the first ARPANET network email was sent, introducing the now-familiar address syntax with the ‘@’ symbol designating the user’s system address. The Simple Mail Transfer Protocol (SMTP) protocol was introduced in 1981.
Why is email still relevant?
Emails are widespread in business and personal as well. I believe the main reason why it is still relevant is usability; the simplicity of use makes it effective and easy to understand.
Software technologies that have been around for a long time present the following characteristics:
- Reliable and high performing across decades makes it hard to justify the change
- Portability and efficiency makes it easy to spread in many solutions
- Simple to use gains users attention and produces a network effect in which if you have it, I want to have it
- High levels of abstraction between what you want and how to do it
- Interfaces offer high levels of abstraction between computers, computer programs and organisations
- Abstraction and decoupling offer great longevity to resolve problems due to the ability to replace the implementation without impacting the abstraction itself
Which technologies do you believe are still relevant? And what do you think it is the reason?
-  Software Technologies that Stand the Test of Time - GOTO 2021
-  IBM mainframe from Wikipedia
-  Why the mainframe is alive and thriving by Ray Wang
-  SQL from Wikipedia
-  SQL is 43 years old — here are 8 reasons we still use it today
-  API from Wikipedia
-  C (programming language) from Wikipedia
This is a personal article. The opinions expressed here represent my own and not those of my employer.