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Backwards Compatibility is pointless thing to watch on..
We are discussing namespaces, need of rebuilding the core of CI, and Backwards Compatibility will be not possible at all..

Backward compatibility is never pointless. It doesn't matter if the core is completely rewritten from scratch. It can still be somewhat compatible without requiring us to rewrite our applications. Your quest for the bleeding edge PHP programming is not worth the time it will all takes us to rewrite everything. What's the point in using a framework if it can back-stab me anytime and render all my previous work worthless. Maybe Laravel is better for you.

I don't say that ci4 should have no BC.. But CI needs some power. If we focus on keeping maximum BC you will have result as IE 10 .. New which is already old ..

This is major release until it there will be CI 3 versions perhaps 3.1 /  3.2 ..
You are can to keep your apps on CI 3 no body is forcing anyone to upgrade..

BTW I am using CI, Laravel and Symfony and I will be very happy if CI 4 will be the one in my future projects as its hard to know all small tricks and details of 3 frameworks at once.. 

You may not have said it should have no BC, but you did say that BC "is pointless thing to watch on"... IE is an interesting point for illustration:
1) IE will be included in at least some versions of Windows 10 for backwards compatibility
2) IE and the new browser will share rendering engines, making the new browser backwards compatible with all of the crusty old non-standard HTML, CSS, and JavaScript that people complain about so much.
3) The compatibility break in the new browser is with the plugin functionality of IE, so the break is limited in the number of impacted users, developers, and sites, especially since most of the widely-used plugins have already been rewritten for the new browser or most of the sites which used them were rewritten to not use them around the time it became popular to make your sites work on mobile browsers.

The idea that no one is forced to upgrade is simply not based on reality. I'm currently in the process of upgrading my site to CI 3 because CI 2 will no longer receive security updates after October. Even with a significant amount of backwards compatibility between CI 2 and 3, I'm still looking at a significant amount of work in some portions of my website. If CI 3 has continued security updates for less than a year after the release of CI 4, and the backwards compatibility break is more significant than that between CI 2 and 3, I would really consider just moving to something else. In fact, if I didn't feel that there was at least a possibility that CI 4 would have some level of backwards compatibility, I would be looking at starting that process now rather than spending so much time trying to get my site ported over to CI 3.

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