The Max would be better described as an all-new aircraft, disguised to look like a 737 in the hope that the FAA would fall for the ruse and allow Boeing and the airlines to train the flight crews with nothing more than a differences course provided in an iPad.
The Max is no more a 737 variant than a Mack Truck is a variant of a Ford transit.
Basically, when Boeing came to refresh the 737, they wanted to provide significantly improved fuel economy, to counter the much newer and better performing aircraft from Airbus. The problem? They could only do that by fitting new engines. Engines that are so much larger than the previous generation that the Max had to be fitted with extended landing gear to stop the bottom of the new engine cowlings scraping along the runway.
But the problems ran so much deeper than that. The larger diameter engines moved the centre-of-mass of the engines lower. Which moved the centre-of-mass of the aircraft. Which meant not only that the flight surfaces were the wrong size and in the wrong place, but that the entire flight geometry of the aircraft was screwed up.
In a conventional airliner, the flight characteristics [in flight trim] should be level flight. Let go of everything and the aircraft should jut fly along, straight and level, until the fuel runs out. With the Max, that was impossible, because the mass of the new engines screwed up handling. So Boeing invented "MCAS" to generate a completely fake flight characteristic for the aircraft. Just like the F-16 is fly-by-why and inherently unstable [cannot fly without the computer], so the Max used MCAS to keep the thing stable when it would otherwise not be.
That's why the partial failures of the MCAS system caused such catastrophic failures, even when the pilots followed the correct procedures.
There were other issues, for example the MCAS design changed significantly during development, but when the plane was tested for certification, the old design limits were tested, not the new, more aggressive ones. And of course the inspectors were Boeing employees, recruited and paid for by Boeing, not independent FAA inspectors. In other words, serious regulator capture issue, right there.
Ultimately, though, the crashes were caused by greed. Airlines didn't want to pay the additional cost of developing an all-new replacement for the 737. Threatened with loss of sales to rival Airbus, Boeing chose to cut all sorts of corners and basically ended up pushing an old design too far.
In the heart of Boeing are engineers and designers who are brilliant at what they do. These issues aren't "engineering-led", but "management-led". Congress should order the FAA to step aside and have the relevant House Committees conduct a thorough hearing. The House should demand access to internal emails, summon witnesses from engineering and management and fully understand how the series of questionable decisions that led to the Max being certified came to pass.
But there's an interesting fly in this ointment. Unlike aviation regulation agencies elsewhere in the world [say the CAA in the UK as an example] the FAA has two roles. One is to inspect aviation and ensure that it remains safe for the public. The other is to promote the use of aviation to the nation and the world. Those two missions are in uneasy conflict.
You only need to look at recent history to realise that the FAA were one of the last agencies to order the aircraft grounded to see the impact of poor governance.
Bottom line is that Boeing are too important for balance-of-trade to take a serious Congressional hit for this. Especially with the serial failures the current administration have with trade deals. Expect this one to be swept under the rug in due course.