I’ll go a step farther: improved frames and datasource-aware tables and lists with a few very basic features found in almost any other UI kit out of the box would have given us 99% of the actually-beneficial stuff AJAX did, but better.
The Web is a ton worse because we decided to build apps on it but never built the tools to do it right, even though the building blocks were right there.
IMO the biggest problem with the way frames works is that it doesn't work well with navigation. I think unfortunately that this is just a design flaw with frames and it needed breaking changes to mitigate.
I think I would've rather seen it go that direction, but it's hard to say. Without a crystal ball, we can't really compare the outcomes, and it's hard to imagine what would've happened in this hypothetical. I mean, I don't think in 2004 I would've been able to guess (or stomach) what the web was going to become 20 years down the road.
The Web is a ton worse because we decided to build apps on it but never built the tools to do it right, even though the building blocks were right there.