But Isaiah was a prophet.
Besides, some of Israel was expecting a political Messiah, not all of Israel. Jews were not a theologically uniform community and there were plenty that had a mystical understanding.
Here’s the thing. The concept and the understanding of ‘messianic expectation(s)’ really underwent a sort of evolution as time went on. At first, the expectation was really political: ancient Israelites expected an (ideal) Davidic descendant to become king and usher in a sort of golden age. Some scholars would say that this belief ties in with the idea found in the creation stories in Genesis that Yhwh, who is the true King, entrusted man to rule over the earth as the human representation of His kingly rule. Adam, however, proved to be an unwise king, and his dominion was taken from him.
By the time of the early monarchy, the ideology of the Davidic dynasty claimed that God is restoring His reign over creation through David and his royal house. However, as you may know, the unified kingdom was split by internal conflict, leading to the creation of the kingdoms of Israel and Judah. That however didn’t stop the Israelites from believing that one day, one of David’s worthier descendants would step in and usher in the golden age when Yhwh’s dominion over the earth would be established and everything would be restored to Edenic innocence.
It’s really in this context that Isaiah was speaking of the son who is born who will have dominion on his shoulders. While (as we know now) it ultimately refers to Jesus, for Isaiah and his contemporary audience it also had an immediate meaning. After a period of crisis which threatened to bring the Davidic dynasty to extinction, an heir was finally born: the future king Hezekiah. It seems that many Israelites (and probably Isaiah too) hoped that Hezekiah will prove to be the expected Davidid.
Eventually came the Babylonian Exile, which again threatened this expectation. It proved to be tough to kill, however: it evolved.
It is with Ezekiel that we begin to see a reinterpretation of the idea: the royal element is still present, but the Davidic element is not. (Apparently, Ezekiel didn’t keep his hopes high that the Davidic dynasty will be literally restored.) Instead, for Ezekiel the historical David is a figure of the ideal king who will save Israel: whereas heretofore the idea was that the future king will be a literal descendant of the David, now the expected one is seen to be a new, ideal David himself, who does not have to be necessarily related to the historical one.
Years later, the Jews returned from Babylon. They were governed by Zerubbabel, who was the grandson of Jehoiachin, the penultimate king of Judah. The David expectation was still in the minds and hearts of the Jews: will Zerubbabel be the one we’re expecting? Now of course, the expectation had an added layer of meaning: the expected Davidic descendant will not just bring about the golden age, but will also restore the Davidic house to its former, royal glory. Ezekiel seems to have left his mark on the Jews: it seems that Ezekiel’s ideal was for Israel to be governed by two heads: the ‘prince’ and the priest, and for the returning exiles, Zerubbabel and the high priest Joshua fit the bill.
However, while he did some things of note (such as restoring the Jerusalem Temple), ultimately Zerubbabel failed to usher in both: Judah (aka Yehud) remained a Persian province, the Davidic monarchy was not reestablished - in fact, even Ezekiel’s ideal of a royal-and-priestly government was not fully implemented; in fact, by the 450s BC, only the high priest remained as a figure of authority. Another false start.
In the writings of the prophets who lived in that time like Haggai or Zechariah, you can see the shift in thinking occurring: while Zerubbabel is eulogized (God calls him His “servant” - the same term applied to David - and promises to wear him like a signet ring, a symbol of authority), but at the same time, these prophets look beyond Zerubbabel: the expected king is still in the future.
It was around this point that the ‘Davidic expectation’ took a backseat as the Jews began to settle down. There was still the expectation of a better world and the restoration of Jerusalem to its former glory of course, but emphasis is now on human activity: we can see this attitude reflected in the book of Judith (9:8-13). Joshua ben Sira (aka Jesus son of Sirach), writing in the 2nd century BC, did not include any hint of an individual messianic figure or anything resembling this in his book, though there are impressive references to the divine upholding of the Davidic dynasty (Sirach 47) and the Aaronic priesthood (Sirach 45), as well as to a general hope of deliverance (Sirach 35:18-19; 36:1-7). For Ben Sira, hopes of ‘messianic’ expectations by the hand of man have already been clearly realized.
