This is engineering, there's a lot of things that could happen but don't, we don't all run ECC RAM either. The problem is that speculative execution is really good and if Intel didn't have it they would've been selling worse CPUs. And to be clear, it was about 20 years from the point where people were seriously publishing theories about speculative execution attacks to the point where it was a practical attack.

Think about how much benefit we gained during that time. And even then, anyone running in a trusted environment would rather have the optimization consequences be damned. Do you think HFTs patched their boxes to criple their perfomance? No.

Sure, now we know it's a problem we'll offer solutions for people who really need it. But it'll be a long while before the average person needs to think about this and in the meantime billions of people benefitted from better CPUs.