It's a crappy old airframe because it's designed specifically to be low down to accommodate airstairs, meaning it won't fit modern large engines. It's anachronistic design and has to make tradeoffs in order to stay relevant (even before the MAX, the NG had those weird squished engine nacelles).
The crazy workarounds are maybe bad designs, but an airframe design that's still in demand after 50 years and been built 10,000+ times isn't a crappy design.
I think you'd be hard pressed to find anyone familiar with aviation agree that the 737 is a poorly designed airframe...
It's only in demand because of inertia. This is like claiming that Windows is great for no other reason than because so many people use it, or that the US government is a great idea just because it hasn't collapsed yet.
If you think it's so great, then why don't you explain why you can't put today's large high-bypass turbofan engines on it in the normal position without them hitting the runway? How exactly is that a "great design"? It may have been a great design in 1967, but it isn't now, just like a 1967 Mustang chassis may have been OK then, but is totally inadequate now.