The decisions about monopoly were not just made in hindsight, they were made in the context of Judge Penfield Jackson's definition of the market that SPECIFICALLY EXCLUDED both Apple and Linux as they were uncompetitive.
But as you appreciate, usually when a criminal is trial, the argument is about whether or not they did X [illegal act]. With anti-trust, the argument is not whether the criminal did X but whether X is illegal or not. It's always opinion.
Either way, Microsoft should still be judged entirely on what it did in 1995. It's not like the world has changed since then ;-)
What qualifies as an abuse of market power? That's up to the Feds to decide, there is no set objective definition that can be run like an equation, such that you can be 100% sure you're breaking the law. It's extremely difficult to know beforehand, and the only safe way to approach it is to either directly ask a federal oversight committee (which many companies do once they're in such a position), or to shy away from any kind of aggressive competitive behavior.