The "support" most importantly includes security updates. You better bet every hacking group has been working at finding fresh zero days for Windows 10 and is stockpiling them to start hammering any PCs that can't be upgraded this October
In fairness, after October that security system will still be in place. The difference is that as soon as attackers finds a bypass, the security system will be worthless against future threats
I wouldn't be surprised if there are some exploits that have already been discovered that people have been sitting on in anticipation of support ending soon.