Actually I am an Engineer, thank you, if it actually had any relevance here.
Nobody is arguing with you that the stacked box frame is stronger than the I-beam, that goes without saying, but it doesn't mean that is the ONLY way to build a fifth wheel. As you say, it's expensive, and it raises the floor to a nosebleed level, (we won't mention the rust out potential).
You are a Lippert slammer, and have been for years on this forum, and that is OK, but we all don't have to drive a Bentley man! I'm not a fan of Lippert as a company either, but the 12" Lippert frame design has proven out so far to be reliable, without the additional cost of the tubular stacked type. We all don't have to drop Bentley money on a camper to enjoy camping. We all don't have to live in a gated community. Getting kind of tired of you looking down your nose at anyone with anything less than what you think is perfect - that's called arrogance!
Carriage mismanaged their company and they were liquidated as a result. Perhaps selling their name to Crossroads was the last mismanaged piece of their heritage they left behind for the prior owners! I personally would not have jinxed my Crossroads product by putting that name on it, but that's just me.
PS, we looked at Lifestyle when we were shopping. They were nice, but kind of unremarkable. Styling was pretty dated with the flat front, and this one only came with a queen bed and 2 dr fridge, so we moved on. It's not always all about the frame.