Actually I was pointing out a critical flaw, and that's Multisite is hard
WordPress should be rebuilt ....
That's unrealistic. It's not that WP is scared or anything, it's that it's a function of it's time, and while it's being worked on and changed and upgraded over time, it's never going to be totally, 100%, rebuilt from the ground up. If it came to that, someone would just make a new product, like Ghost.
WordPress was not 'threatened' by mysqli and dropping support for PHP 4. WordPress (which will be 11 this year, not 8, and was a fork of b2, which started in 2001 making it 13 this year, so we predate mysqli anyway) is actually pushing for hosts to drop PHP 5.2 so it can force everyone to move forward. WP would love it (and checks constantly) if it can drop support for older versions of PHP and move forward to the newer ones all around.
but i think the network section can be improved...
Yes, and improvements are happening. In fact we had a Multisite chat last week, and will every week going forward: https://make.wordpress.org/core/2014/05/13/agenda-for-multisite-chat/
But all the answers depend on what you mean by putting Multisite first.
lot of plugin developers have to write code specially for networks, they have to fill their code with different conditional methods to check for things that can be forgotten if the platform is well architectured.
If you know how to fix all that in Multisite, please do come to the chats. It's Wednesday in IRC at 6pm UTC and we would love for more developers who can see how to make that happen to show up :)
Just don't except anyone to jump on the 'rewrite WP!' wagon.