There are a bunch of different angles to consider the evolution of a spreadsheet, and, as I say in my response above, I personally think focuses on changes to the UI/display of data miss the point: what's missing in Excel 1.0 isn't a better display of data - IMO, it's giving the modern, powerful analysis tools that us programmers have access to the beginner-end of the programmer spectrum!
Different spreadsheet startups certainly have different theses on this. Subset [1] (the OP) seems to focus on side-by-side grids on an infinite canvas. Monday [2] (also referenced by OP) seems to focus on different "views" for a spreadsheet for project tracking, etc. Mito focuses on allowing you to integrate Python and spreadsheets as easily as possible. Clay [3] seems to focus on spreadsheet integrations into APIs/other data.
(Disclaimer: all the above are just my understandings of these tools, but I haven't used most of them directly mostly am just going of marketing materials... I highly recommend you check them out, though - they all look quite cool!)
My post def was a plug for Mito - I'll try and make my response to the post/thesis more clearly delineated in the future. I think this post is an awesome chance to get feedback on our spreadsheet thesis (and potentially hear back from OP on this thoughts!).
Rock on, spreadsheets :-)