I agonized over this for a while before I decided that Britax was no longer allowed for my kids. My real issue is that if this happens at 35 mph, what about 45? 55? We live in a big city, and I'm going over 40 mph almost every trip, and frequently over 70mph, in the city. I switched the Frontier to a booster as a trial and it was terrible- a very bad fit. It's probably better for a much older kid, but mine isn't huge yet, at 45 lbs and 5 years old. We removed that and put the diono radian in, but it wasn't compatible FF in the available seat. We ended up with a HBB that fits him well, but it still makes me nervous. I had to weigh how nervous I was with that versus how nervous I was flying down a highway, and the booster won out. I'm an engineer and have watched my company run crash tests for what I consider a very same car company, so I've seen how big of a deal a few miles per hour is. My opinion was he is safer in a properly fitting HBB than a harness that has questionable results. Also, seat belts have been crash tested at more speeds than car seats have, so I trust them more. I used to not but I assumed car seats were tested at much higher speeds. It's pretty upsetting that they aren't, and I'm glad we never had an accident while using them.
Depending on where you live and how you drive, you might come to the opposite conclusion and be right for your circumstances. There's no right answer unfortunately.