This was perhaps the case in the very beginning, but everything is switching towards the built-in completion interface as a core, where you can extend both ways

This was perhaps the case in the very beginning, but everything is switching towards the built-in completion interface as a core, where you can extend both ways: provide elements to complete, or provide ways to perform the completion. What you get with ivy/company/etc is different ways on how to expand/handle/show completions and they're all still relevant. I'm using different frontends depending on the mode that I'm using. I personally dislike pop-up style completions. I'm using a mixture of completion buffers (the boring emacs built-in ones) ivy and ido instead.