Nitpicking specifics misses the elephant in the room:
> figure out a sane way to do reactivity
(100-epsilon)% of developers write websites in a reactive framework, and the reason we have so many of them is because the platform doesn't provide a sane way to do it, so we keep trying again and again with different hacks on top - virtual DOM, proxies, literal JS to JS compilers, whatever some crazy guy happened to come up with last weekend.
That's the problem developers have, and WebComponents don't solve it. They can't. Nothing you do in the DOM can solve it, you need support from the JS side. Thus, lock the two working groups in a basement until they start talking to each other.
All developers want to be able to do is write this:
JS:
let my_stuff = [1,2,3];
HTML (made up template syntax, pick your favorite one)
<ul><li><$for thing in my_stuff><li>$thing</li></$for>
JS again
my_stuff.push(4, 5);
my_stuff.reverse(); // the user now sees 5 4 3 2 1
A giant update that forces everyone to change to drop their workflow, but doesn't solve this was bound to be dismissed oturight.