When did the term 'microcomputer' become obsolete?

Solution 1:

Microcomputer originally differentiated them from mainframes (AKA "big iron") and minicomputers.

Minicomputers declined in the mid-80s and became obsolete during the 1990s. Mainframes remain in some use, but much less than they once had, while completely new types of larger computer system exist.

Differentiation of micros from mini and big iron is less useful. Meanwhile differentiation between types of micros became more useful, first between "home computer", "personal computer" and "workstation" in the 80s, later such differentiations as desktop, laptop, notebook, various distinctions within small microcomputer servers, and so on.

At the same time, the introduction of the microcomputer meant that computers changed from being something most people had heard of, to something most people increasingly saw and increasingly used themselves. With microcomputers being the type almost everybody knew about, and those using other types still mostly using microcomputers more often than any other, they became what people thought of when they thought of computers.

Both of these two changes that made microcomputer a less useful term started around the mid-80s and were largely complete by the mid-90s.