Scams, stealing credentials, stealing money, botnets, viruses, losses of data, ransomware, etc etc etc.
What is better for most people is a locked down device like an iPad where each app has to be approved and they're incredibly sandboxed. 20 years ago we had people installing malware because a strange email promised them smiley face emojis.
When we transitioned from the single-user ODS-based Windows model (ie Win98/SE were the last of that line) to a multi-user restricted privilege model based on NT 3.0/3.5/4.0 (first as WinXP) it was meant to be better but privilege escalation was still too easy because of what users had become accustomed to doing and of what was needed to install software you downloaded.
Things like an App Store (on Mac and eventually on Windows) are actually a good thing. Signed apps are a good thing. Having to go out of your way to install unsigned apps is a good thing.
I really abhor "technical libertarians" because they never address these issues. It's all principle-based while ignoring reality, human nature and whether or not unfettered access gives users something they even need.
Also, other people pay the price. Where do you think these DDoS attacks come from? Compromised Windows PCs (primarily).