The article mentions about security issues like XSS and XSRF, and yes these are legitimate issues. But again, it's a developer problem, like SQL Injection before it, it's a common issue that developers have to actively make sure they're doing their jobs properly to minimize security problems (The reason I didn't say ELIMINATE is that there's no such thing as perfect security... although most security consultants would of course tell you otherwise)
ABSOLUTELY true, ESPECIALLY true if you're implementing your site as a Single Page Application (SPA) Google has some guidelines on making your AJAX links indexable but that probably only works with Google. Surprisingly the solution I used which was to create an invisible link to a dummy non AJAX static page with listing of links worked pretty well, I later found out that this was an actual method which people have talked about called a snapshot.
"Client-side technology is doomed to fail" Wow... that sounds exactly like how people use to argue that ThinPCs would become a thing and normal x86 desktops would fade into obscurity. Let's break down the original article's examples.
Java applets failed primarily because the additional cost of the runtime download made it a barrier to entry for normal users BEFORE the broadband age, although once we DID have the bandwidth Oracle didn't seem to be interested in paying much attention to the applet usage of Java anymore. It also didn't help Java's case that a HTML5 browser's capability has caught up to MOST usage scenarios for a Java applet.
During it's time Flash had a smaller runtime download and could deliver impressive multimedia capabilities and features when compared to Java, that's what made it the dominant plugin previously. There's a chicken and egg argument about the role Apple played in Flash's demise when they choose to chase the HTML5 standard instead of Flash, but without a doubt HTML5 once again had most of the important features which someone would use Flash for but WITHOUT the additional runtime download.
There's only one thing that Flash is currently still ahead compared to HTML5, and that is it's ability to manipulate and playback video files. Due in part to everyone still trying to figure out how to standardize video manipulation based on patented video codecs I believe.
And that's it... damn.. this rant is longer than the original article, well, hope it helps to provide some insight to this topic!