It's not after the fact.
Why is it ok for something like the unimog but not for a tundra/Tacoma?
This super stiff frame thing is only a fad. The way you guys think, the Colorado frame is stronger than a 2014 f250 or f350. After all they had c channel.
A stiff frame isn't stronger. It's only stiffer.
The hmmwv frame is boxed but it's flexy. The frame on my land cruiser is boxed, but it's flexy. I guess the Colorado frame is stronger than a land cruiser frame.
The Tundra/Tacoma are no where close to being in the same ballpark as something like the Unimog when it comes to frame and chassis design. Go read some articles about the Unimog; it's chassis, engine mounting, body mounts, ect. were all designed to accommodate massive frame flex. The Tacaom/Tundra were designed with a c-channel frame, and the engineers just accepted there was going to be a bit more flex inherent to that design.
As for stiff frames being a "fad," that setup has actually become the norm for modern vehicles. All of Toyota's global 4x4 platforms use boxed frames. Just about every BOF SUV or pickup in the North American market uses a boxed frame, including the new 2017 Ford Super Duty. The only North American pickup's/SUV's that I can think of which don't use a boxed frame are the Tacoma and the Tundra/Sequoia.
All frames flex, some to a lesser degree than others. But the general consensus is that you want the suspension to do most of the flexing and articulating while the frame remains mostly rigid. That's why most offroad-focused platforms, like the Land Cruiser, use boxed frames. The Unimog is the exception to that rule, but again it was designed to have a flexy frame/chassis, unlike most other 4x4 platforms.
2013+ vehicles aren't that old yet. We are only half way into 2017, get back to me in when there is is another 7-12 years on them and we will see how well they faired. Hell, lets see them when they are 18 years old like my current Tacoma.
Yep, they're still young, relatively speaking. But I don't think you even know what you're complaining about at this point. You brought up a lot of issues (turbo's, fuel pumps, injectors) that are potential pitfalls for any modern diesel (including the diesel used in your esteemed Hilux). Heck, turbo's are a potential pitfall for gasoline engines as well. The new gasoline engine in the Tacoma is largely unproven in a truck platform. You have to 'wait and see' with any new vehicle design.
But oddly enough, you only seem to vocalize your skepticism when it comes to diesel vehicles.