I don't see any reason why you can't have the agility of Excel with a few of the nice tools that modern IDEs have to make it easier to debug or secure a spreadsheet "app". Excel actually has a lot of features for preventing issues such as formula debugging, input validation and named ranges that make formulas more readable, but the UI is terrible and it's not very discoverable. The problem with Excel seems to be primarily that the MS desktop monopoly made it very hard for people to market better UIs for the "quick financial model" use case. The only way around that was either in B2B sales (where the economics forced you into building big ERP systems where management locked down the processes) or over the web, where we've had to wait for browser technology to reach a sufficient level of power before such a system could be built. I think since IE9, we're now at that point. It wouldn't surprise me if we now start to see a multitude of apps chip away at Excel's dominance in each of its use cases (like trello is doing for the 'lists of stuff' use case).
We (Fivetran) are doing exactly this, bridging the gap between spreadsheets and coding and making real algorithms accessible to non-programmers. The primary thing that makes spreadsheets more approachable is the live-updating, and this feature can be separated from the grid-of-cells model with a cleverly designed language. We'll be starting a private beta in the next few weeks, stay tuned.