1) The case's lack of merit was not my opinion, it was in the official ruling:
After thousands of hours of testimony (testimony of over 950 witnesses, 87 in court, the remainder by deposition), and the submission of tens of thousands of exhibits, on January 8, 1982 the anti-trust case U.S. v. IBM was withdrawn on the grounds that the case was "without merit."
https://www.historyofinformati...
2) The case wasn't dropped until 1982 by which point any monopoly in mainframes was increasingly meaningless.
3) IBM was a old boys club and the board of directors did not recognize the value of the personal computer market. The team developing the PC likely did but they were not given adequate resources by the board.
4) So you admit they thought the market was not super valuable.
5) It might be one reason but certain not the main one.