IVR, IVA & human agents -- choose wisely

Every day, call centers process thousands of calls using live operators. This has a number of disadvantages such as customers spending a long time waiting on the line, low customer satisfaction and a high cost of service relative to other means of communication. The growth of popularity of voice assistants and other NLP (natural language processing) solutions like speech analytics give us a feeling that we can totally automate contact centers without any negative consequences. But what are hidden pitfalls? And how not to get to slippery slope? IVR and IVA -- what and when to use? IVR (interactive voice response) are very popular from 70s.