The '6 hidden costs' can all be addressed, some are not even hidden like time, running tests take time and resources like anything else. The other factors like speed/performance and confusion can all be solved very easily.
Some of my take-aways for testing
- Don't focus on small changes like buttons, copy etc.. these are mostly useless yes
- Make radical changes, e.g. on stickermule can try to have the whole product to make a sticker to checkout on 1 page or start with configuration first and then select sizes etc..
- To filter out false positives, compare same sample of data (e.g. organic traffic is good) for the same period range before a winning test went live and after
Personally I have seen some great results with the above tips in mind, from B2B making visitors perform small actions first or showing a demo vs. presenting them with a form immediately (4x more leads) to e-commerce sites adding a direct checkout vs. normal checkout path (lower avg. order but much higher conversion rate to largely make up for it) etc..
You could argue these are part of product (which a conversion department should be part of, not marketing) but without doing the testing you can only guess about the outcome. And sure many tests are fixing usability (e.g. seen 250% increase in registrations by fixing telephone number prefix and date of birth field formatting) and UI issues but that's part of it.