We're already seeing some ad networks using initialization scripts to bootstrap their ad setup, with dynamically constructed secondary scripts that have a high fetchpriority (so they supposedly load faster than others).
For reference, modern news/media sites may load multiple ad networks (3-4 on average perhaps) in order to optimize ad performance (revenue) or inject different kinds of ads or sponsored content. If we take Google's Ad Manager (or similar) as the base network to serve ads (own or third-party), many times we see additional networks like AdSense, Taboola, Outbrain, Vidverto (to name a few) or other local ad networks that do some sort of header bidding, ad injection, sponsored content display, in-read ads/videos and so on.
I don't see why some won't abuse this spec to force-inject their stuff earlier than other ad networks and of course at the expense of a site's performance...
On the other hand, there is no real reason for a web app to have jumping content as is - it’s just a bug in said app.
So... it will be on every website.
It'd for sure still be up to content designers to degrade gracefully.
There's an HTML "<noscript>" tag that works perfectly well...
However, it's sometimes a necessary evil due to:
1. The original company not budgeting time/effort from their existing dev teams to work on A/B testing, and wanting to outsource it instead.
2. Said company and their tech department being nervous about letting outsiders actually access the source code for their site/app.
3. Or said company wanting the analytics/features that VWO/Target/Optimisely/whatever offer, and not wanting to have code up the same analytics toolbox themself.
