Why the assassin rework is a failure and the new tank meta

Hiya. So with the next assassin rework coming around the corner, I'd thought I'd take the liberty to post my personal opinion on all of it. So from what I've gathered, the assassin rework is designed to two things:

1. Lengthen the burst sundown of assassins
2. Make assassins unique to one another. The most significant problem I see here is the first one. Lengthening an assassins burst window. So why is this such a problem? Firstly, believe it or not, an assassins job is to one shot. How exactly they one shot carries is basically determines the greatest assassins from the worst. Which assassin can one shot someone 9/10 times and escape while alive? This makes champions such as fizz Leblanc and zed problematic. Their ability to one shot while remaining completely safe. So, since an assassins core job is to one shot, the only way to balance this is to add counter play their one shots, or make them less safe while doing so. The worst way to balance assassins is to "lengthen their burst window". Let's see what's happened in the past when riots attempted to do this. Probably the best example of this is ekko. The classic assassin built tank. The reason he works so well as a tank is precisely because of his "burst window". His 3 hit passive, to be exact. It's much easier to proc this passive when built tank. In conclusion, the main problem is, if your going to lengthen an assassins burst window, why not just build them tank? The whole idea of Ann assassin is burst damage, and the whole idea of tanks is sustained damage. When the lines start to blur is when problems arise. So why not just build the reworked assassins tank? Tank rengar has already been shown to be a possibly sleeper pick, and tank akali is expected to be up and coming.