I doubt many people are wanting to build a modern app with CI3 now that we have CI4 and "other" alternatives.
But there are going to be many legacy apps built in CI3 which still work perfectly well and achieve their purpose. Let's say you have 20 projects built with CI3. How are you going to convince every client to go through the huge process of having them completely re-written in CI4? And those CI3 apps might not be that old, given that CI4 is pretty new. And even if you did convince all 20 clients to have their apps completely re-written, what do they gain at the end of it? Not a great deal I would say. So this is why people would like to see CI3 still maintained and supported.