You seem to have a perception that there is a rigid, correct process which must be followed. This is not the case. There are many different schools of design processes, and even within a school, there may be many different sub-forms. In other words, everyone does things differently, depending on how they make sense.
- Do you have to start with user interviews to identify needs of a target audience? No, there may be other research out there, and you may be developing something so radically different that nobody quite understands yet that they need it (radio is a good example here - nobody asked for radio, operas and newspapers already existed). But it often is helpful to interview the exact people you're developing for.
- Do you need to keep your initial interviewees around? No, often they'll drop out anyway as their life moves on. It can be helpful to keep them around to validate that your needs synthesis actually is vaguely in line with what they had expected when making their answer, but you probably aren't designing for any one of the initial people directly - you're designing for a market segment, for a larger group of people than you had interviewed initially.
- Do you need to ditch your initial interviewees to recruit new ones? Also no, you can have as many as you want. There are practical limits depending on your budget/interviewer work load, sure, but no conceptual ones. Also note that it doesn't have to be all-or-nothing, you can for example ditch 50% of your initial folks and have 50% new ones.
- Do you need to rely on user interviews to identify holes in your product? No once more, you can get incredibly far by reviewing what your competition does well, or by actually using it yourself (and with your QA engineers). It can be helpful to show your users a half-finished product, but depending on your app and progress, it may be that your users won't understand what you're trying to do until they actually can see a fully functional app (or prototype).
So, all in all: Everything is optional. Some of it is sensible to do in many situations.