There's way more to it actually.
The Flight Computer as architected was found to be a single point-of-failure with a catastrophic failure mode with the software as delivered. That's a big fat no-no. Even if it was only under the most unlikely circumstances possible, you don't design a system that can fail that way, even in theory. Murphy is considered to always find a way, so you must be able to survive that casualty.
Therefore, they had to rearchitect the software with greater redundancy, which in aerospace, comes with such a load of verification burden as to be a majority of the work on it's own, especially given as it will almost guaranteed change the operating interface procedurewise in some way for pilots. Throw in the issues getting all international regulators independent buy-in, and fix wish lists, negotiating through those, documenting and publishing training and maintenance material updates...
https://www.seattletimes.com/business/boeing-aerospace/newly...
There were also deeper problems from at least a reading of regulations from my point of view with how interpretations of statutory regulation have evolved, but unfortunately, those are beyond any ability of me to confirm or comment on ongoing developments as a non-industry insider. (Read: practically just me being pissy because no one in the industry except maybe WalterBright has given me a sufficiently convincing reckoning of.) From my recollection of my digging at the time, there were various statutory test criteria that an MCAS-less MAX could not meet, and thus should not have been certified as a civil transport aircraft if evaluated strictly to the statute as written. Obviously, reading of statute doesn't directly translate to 100% accurate assessments of how it is enforced, as they are written to be understood and convey meaning within an intended audience of specialists (which is it's own problem in my opinion), but nevertheless, back in the bad old days, there were quite the number of test pilots who felt that as time went on, the loosening of airworthyness standards from "strictly as written" was resulting in a slippery slope which was resulting in aircraft that were increasingly less capable of being fully understood by pilots, and potentially more dangerous in the event of substantial problems developing with their automation systems.
Obviously, we can see how history resolved that debate, but our forebearer's on the losing side of that argument sadly do win their "I told you so"s.
See the D.P. Davies Interviews with the Royal Aeronautical Society for more on that and some good listening.
https://www.aerosociety.com/news/audio-the-d-p-davies-interv...
https://www.aerosociety.com/news/audio-the-d-p-davies-interv...