Languages become legacy languages because of their applications

Programming languages have brief lifespans and a limited set of destinies.

COBOL, invented in 1959, was considered passé in the microcomputer age (1977 to 1981, prior to the IBM PC).

Fortran, from the same era as COBOL, saw grudging acceptance and escaped the revulsion given COBOL, possibly because COBOL was specific to accounting and business applications and Fortran was useful for scientific and mathematical applications.

BASIC, created in the mid-1960s, was popular through the microcomputer and PC-DOS ages but did not transition to Microsoft Windows. Its eponymous successor, Visual Basic, was a very different language and it was adored in the Windows era but reviled in the .NET era.

Programming languages have one of exactly two fates: despised as "legacy" or forgotten. Yet it is not the language (its style, syntax, or constructs) that define it as a legacy language -- it is the applications written in the language.

If a language doesn't become popular, it is forgotten. The languages A-0, A-1, B-0, Autocode, Flow-matic, and BACAIC are among dozens of languages that have been abandoned.

If a language does become popular, then we develop applications -- useful, necessary application -- in it. Those useful, necessary applications eventually become "legacy" applications. Once enough of the applications written in a language are legacy applications, the language becomes a "legacy" language. COBOL suffered this fate. We developed business systems in it and those systems are too important to abandon, yet also too complex to convert to another language, so COBOL lives on. But we don't build new systems in COBOL, and we later programmers don't like COBOL.

The true mark of legacy languages is the open disparagement of them. When a sufficient number of developers refuse to work with them (the languages), then they are legacy languages.

Java and C# are approaching the boundary of "legacy". They have been around long enough for enough people to have written enough useful, necessary applications. These applications are now becoming legacy applications: large, difficult to understand, and written in the older versions of the language. It is these applications that will doom C# and Java to legacy status.

I think we will soon see developers declining to learn Java and C#, focussing instead on Python, Ruby, Swift, Haskell, or Rust.