The capability to change how most stuff in IPS works and enhance it with own features/apps isn't going away.
What changed is how things have to be done. Some changes will be much easier, others will be probably annoying as hell for people who are used to how the hooks worked in v4, some will think that it is a huge step backward, but as Matt said, it had to be done, to provide a much more stable and faster-improving code base for v5, because we'll be able to alter or even deprecate methods, method parameters and even deprecate complete (internal) classes much faster, which also 3rd parties and clients will benefit from.
With v5 I'm really expecting fewer "A new community release with some huge under-the-hood BC breaking changes is here, all my apps are now broken and need to be updated ASAP" scenarios.
Code listeners are a quite common pattern used in modern applications, most php framework use this approach today. It's the perfect balance between none capability to change the core behavior via 3rd party code and the too-powerful proxy autoloader which we had in 4.x, which as powerful as it was, had also way too many problems.
I've read about concerns about missing event locations which were common in v3 and that it took ages to have new hooks implemented => Keep in mind that we have now a monthly release cycle, so I'm quite sure it's not going to take now that long to ship improvements as long as they make sense and fit into the general vision of the product.