* Excel really has two major ways of performing computations on data - formulas within the grid and actions upon the grid. Despite the utility of the formula based dataflow model, there are too many operations that have to be performed as one-shot operations via commands (or scripted via VBA). Having formula based approaches for sorting, dividing into bins, etc. would be very useful.
* It'd be nice if Excel cells could contain values other than scalars. (Arrays, tuples, lists, maps, matrices, complex numbers, etc.)
* VBA can be used to define custom functions, but there's a lot of marshalling overhead going to VBA and the programming model is slightly different. It'd be nice to fix both of those issues.
* There's no way to locally bind names within a cell formula, so often subexpressions have to be duplicated. (And I believe they're doubly evaluated too.)
We make a collaborative (Google Sheets style) spreadsheet with Python and R running in the sheet. You can define functions, plot using ggplot, embed numpy dataframes, matrices and all that good stuff. We don't let people use macros, all the code runs in cells because we think macros are too brittle. You can check out the website at http://alphasheets.com .
We're seeing that many enterprises (for example, in finance) that have Excel power users are moving to Python because of limitations like these, and are running into adoption issues because people like spreadsheets so much. That's generally where we come in and provide a bridge from the Excel world to Python through a more friendly frontend.
We're also seeing that Alphasheets can help a lot with shortening feedback cycles on more sophisticated data analyses- Excel is the most popular self-serve analytics tool out there, but doesn't cover cases where you need Python/R/fresh data.
Else its the same thing mentioned in the previous comments. You would build a web app with 5% the features of excel, and the moment somebody reaches use case that can't be solved with your tool, they will have to switch to excel. If they have switch every second time they use your product. They might as well do all their work in Excel to begin with.
You have to be feature compliant with excel and you can't do that on a web app alone.
* see a modern replacement for VBA, dare I say using JS
* be able to share a document that won't break when someone opens it on their computer (even if its using all the excel bells and whistles including external data sources and plugins). Google Sheets by contrast, is just a link.
* be able to use all the amazing features via the web and/or an app
Any .NET language is already modern enough and supported via Add-ins.
I just miss not having them fully integrated into Office, instead of requiring VS + VSTO.
The problem is, Excel has a ton of very powerful features. Many of which Google Sheets doesn't provide. Something like VBA would be nice. I'm aware you can write JS plugins for Google Sheets but the experience is no where near as good. Pivot tables in Excel still smoke Google Sheets.
I think this is why Excel/Google Sheets still dominate and will continue to.
All of these products do one or a few things things that Excel or Google Sheet do, but perhaps they make it little easier for a novice. I think what people don't understand about Excel (and to a lesser extent Google Sheets) is that it's an IDE. A novice can build interesting things, a power user can build incredible things.
The only way to make Excel a thing of the past would be to make a blow-away awesome replacement that does everything excel does but better. There's plenty of blue ocean around Excel and I think each of the products you listed could do just fine.
I would love all of Excel's power available to me but delivered like Google Sheets. That would definitely kill Excel. So far neither Microsoft nor Google seem really committed to this. Google Sheets is nice but just grabs the low hanging spreadsheet fruit. Office 365 is anemic.
If I had the time and funding I'd love to make a true Excel killer that was a faithful recreation of ALL of Excel's capabilities but delivered in a modern way. I'd pay good money for this. I believe many would. Excel may be a dinosaur, but it's still the apex predator.