20 lines of code that beat A/B testing every time
This is thought-provoking, which is good.  However there are significant issues with the approach.1. Real world performance varies over time.  For instance there are typically daily, weekly and monthly conversion rate fluctuations.  Not an issue for A/B testing, but a big issue for this approach if a random switch in direction happens at the same time that conversion fluctuations happen to head in a good direction.2. (This is really a special case of #1 - but a very, very important special case.)  This approach creates long-lasting interaction effects between tests and independent changes.  That requires explanation.  Suppose you're running a test.  Version A is somewhat better.  But version B is temporarily looking slightly better when you make a significant improvement to your website (maybe you started another test that works).  Now you're adding a lot of good traffic to version B (good because of the other change) and very little of the new good traffic to version A.  This new version B traffic soundly beats your old version A traffic.  This correlation between time and website performance will continue until the old version A traffic is completely swamped by new version A traffic.  With only 5% of your traffic going to version A, this can easily take 100x as long as your test has been running - or more.  (Properly constructed A/B tests do not suffer this statistical anomaly.)3. Code over time gets messy.  One of the most important characteristics of A/B testing is that you can delete the mess and move on.  With this approach you can't - it just hangs around adding to your technical debt.4. Businesses are complex, and often have multiple measures they would like to balance.  For instance in a recent test, conversion to click was hurt, conversion to a person who clicked 5x was helped.  A/B testing let us notice that something weird was going on and think about what we really cared about.  This automated approach would make a decision and could have hidden a real problem.5. Many tests perform differently on existing users and new users.  A/B testing with proper cohort analysis can let you tease this out and decide accordingly.  This approach doesn't give you that kind of sophistication.