I actually really like their pricing model. You can buy years in advance, and they occasionally offer steep discounts (I think I bought two decades in advance around new year?). In a time where more and more services offer me no other way than some crappy monthly subscription that I cannot pay in advance and without auto-renewal, this is really much appreciated.
That being said, a lot of your points seem valid - I've started using SN quite some time ago and haven't really noticed any new features since (except for some editor-improvements). Organization is indeed something that probably could use some love, the tags do suffice for me personally (especially with that one extension that lets you create folders with tags), but barely. While I appreciate their stability with regards to UX (it looks/feels the same since forever), the features you mentioned (collapsing, rightclick-menu) would not hurt.
Their 'lack of new features' however is, as far as I understood, somewhat intentional - back when I bought it I've read somewhere on the page that they explicitly have the philosophy to also say 'No' to new features if they think it threatens their guarantee of long-term stability/support. Which I think is a very admirable stance these days where short-term-KPIs seem to dominate entire industries.