Once again I waxed verbose on Quora, answering:
and wanted to preserve my answer here on my own “property”, so:
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First, an important disclaimer: I am not now, nor have I ever been, a member of the lawyering profession. Take everything herein with a large sack of salt, and follow this advice at your own risk. Now:
Technically, in many countries, anybody can sue anybody for anything. Whether they’d win is a different question. That may sound facetious but it is a very important distinction, as even being sued unsuccessfully can really ruin your year.
In this particular case, suppose I wrote a complete replacement for, say, PowerPoint, from scratch. I would fully expect to be sued by Microsoft. At the very least, they would abuse their vast resources to try to grind me down, prevent people from using my program instead of theirs, keep me from enjoying the fruits of my labors, and “teach me a lesson” not to intrude on their “turf”. (Well, at least I would expect it from MS as it was run by Gates or Ballmer. Nadella would probably be much nicer about it, but there’s still a significant risk.)
Even if they lose, it’s just a cost of doing business, just like all those lawsuits and fines and such they’ve paid in the past. Even if we were in one of the more sensible countries, where the loser has to pay the costs of a lawsuit, which puts a bit of a damper on frivolous or abusive lawsuits, my full costs plus theirs, and even any reasonable amount I could countersue for, successfully, would not even be pocket-change, not even couch-cushion change, to Microsoft. If Microsoft were walking along the street and saw that much money just sitting on the ground, it wouldn’t be worth their time to stop to pick it up.
Now, would they win? On the parts most people would think of, the code itself, obviously not. I wouldn’t even have access (so to speak) to their proprietary code, so as to be able to plagiarize it.
But then there’s “look and feel”. If any of those features my program has in common with PP, could fall under the umbrella of “look and feel”, or a unique non-obvious workflow, or anything else MS could legitimately claim to have invented and I could easily see just by using PP, then they may have a legitimate claim! In some countries and at some times, such things have been considered legitimate IP (Intellectual Property). You haven’t filled out your profile, so I can’t tell if this applies to you where you are. They may have actually taken out a patent (or other such legal protection) on such things, or they may be able to make a valid IP claim without such paperwork done in advance.
So: provide the same functionality (ideally more), but make it look and feel different, and don’t copy their workflows and such except where it’s either a common standard or an obvious thing to do, and you should be relatively safe against justifiable lawsuits.