Hi,
There's lots of ways for your current uses to "shoot the cat"; perhaps you've been lucky so far. Or perhaps you accept what they do behind the scenes.
First, MS Office. They deliberately add incompatibilities, forcing you to upgrade (ie. pay them again) so you can open that Word 20xx file from your client/employer/tax man/whatever. Nowadays they're moving to a subscription model, so you'll have to pay monthly to be able to edit and view documents.
Having Flash installed may lead to compromising your bank details, your system, or any other data you care for.
Your phone will likely stop getting updates, or it will get an update making it slower that you cannot remove. All cases leading to planned obsolescence -> buy a new phone. The OS and apps you run spy on you, selling all data they can gather to the highest bidder. If you're lucky, this only results in more ads for you.
We have plenty of examples of closed software being malicious, but not in an overt way. Perhaps they call home. Perhaps they spy on your activities, to make sure you're not trying to cheat or do anything they won't approve of. Perhaps that so chic note-taking app is trying to steal your bank credentials in the background.
If you haven't yet been bitten by anything, you won't be as careful or think of what might happen. Had you had a book of yours removed off your Kindle, your Steam account blocked because you had a debugger installed, your battle.net account blocked because you ran a game in Wine, an important piece of software stop working and demand an upgrade, or numerous other examples of closed sw being not-so-friendly...
All this is just the negative aspects too. How will you fix a bug or add a feature to closed sw? What if the company making it has gone bankrupt, and you cannot even get them to do so?
- Lauri