How they handle multiple monitors is a choice and the memory leaking is a design decision as is the lack of addon api and a system where any crashing addon takes out the whole session.
The only one out of those that's a intentional design is multiple monitors. The other ones are known shortcomings that are just really hard to fix. The memory leak was fixed a few years ago: https://feaneron.com/2018/04/20/the-infamous-gnome-shell-mem...
It is worth noting that the first release of Gnome 3 which had the leak from the outset was in 2011 and in 2018 they mostly "fixed" the problem by just constantly running the garbage collector and marveling at how not so horrible that is.
Unfortunately the problem actually continued for some at least into 2020.
In summary a broken design nearly completely ignored for 7 years and fixed with a band-aid that doesn't actually solve the underlying issue over the next 2 years. 11 years in and the design fail is still with us. I appreciate the fact that many of the contributors aren't paid for their efforts and I applaud their efforts to give to the community but large parts of gnome ought to regarded as learning experiences not something to build the rest of our software on top of.
I hope System76's work will be a better basis especially since their financial success could fund future efforts.
Bugs are inevitable but bad design is not.