Sad story and terrible outcome. Unfortunately from what I have heard the young victim panicked when he went in the water. Not that most people wouldn't.
The waders we mostly wear today are no different than those worn 40 or more years ago. Around the time neoprene waders were becoming popular Fly Fisherman published an article comparing waders including form fitting neoprene and baggy but light latex waders in pool immersion tests. Baggy waders filled with water and made it all but impossible for test subjects to exit the pool. Neoprenes did not plus the buoyancy of neoprenes aided test subjects both in the water and exiting the pool.
Unfortunately most of us have chosen comfort (breathables) over safety.
So much depends on the circumstance when a person loses their footing and goes into the water. It's actually safer to be up to your waist. Slipping and falling head first is a owrst case scenario since the waders will have more entrapped air. It's not unknown for people to float head down, face in cold water and hyper ventilate water into their lungs - usually fatal if they cannot quickly exit or get immediate assistance.
By all means wear a belt - I usually wear two. I think inflatable vests are a good idea though I don't wear one. Once I went into deep water wearing both an old Mustang Floater coat and neoprenes. I bobbed like a cork!
Know what to do if you do fall in and if you can keep your composure; try to float on your back - head upstream and go with the current. Don't fight it. Make a plan how to get back to shore or shallow water. Try angle yourself towards shore and swim with 'snow angel' waves of your arms. Most anglers do fall in periodically. Most survive and most dunkings are trivial and inconsequential.
Be extremely carefully around fast turbulent water and particularly when rivers are in flood stage. It may be best to avoid river banks and wading in such conditions.