Think of the last time you purchased a brand new car. What if it didn’t work after only one day? You jump in your shiny new ride, fasten your seat belt and turn the ignition, and … nothing. The car won’t start.
You might head back into the dealership (if you can find a ride). “I just bought a brand new car from you and now it won’t start!” The dealer shrugs and says “well it worked yesterday didn’t it? You can’t expect it to work ALL the time!”
None of us would accept a brand new car that only works some of the time. Why do we accept that from costly antivirus software?