Unfortunately, I don’t have any proof to say if testers are needed or not. As an outsider, it looks like there are some companies which are doing well without testers and some which are doing badly despite having them. If it is indeed possible for companies to work well with zero or almost zero testers, then I’d like to know their secret and what is the long term viability, especially if the company grows.
I heard that Microsoft has very few full time testers. I’m not sure if they have many temp testers. It seems to be going okay for them. I don’t see too many job postings for testers in Google and Facebook. OTOH, Amazon started posting large number of tester jobs recently. I don’t know if that is partly or wholly due to the pandemic, or also due to realizing that testers are necessary for them.
IMO, to answer your question, we also need to think about the following questions.
What makes excellent testers?
Is it training alone or inherent quality/genetic luck? If it is only due to training, then we could train intelligent people/developers to do excellent testing. If it is only inherent quality/genetic luck, then we cannot produce people who do excellent testing. Then, we’d have to discover such testers.
If someone is an excellent developer, then is it guaranteed that they WILL become excellent at testing?
If yes, then we only need to train the excellent developers to do excellent testing. This won’t be scalable if there is a short supply of excellent developers. If no, then we need to hire testers separately for testing. Does excellence in backend development automatically mean that you’ll be excellent in front end design and UX also, provided you tried hard and spent enough time on it? I doubt it. Similarly, would you be an excellent tester also? I don’t know the answer to that.