I used to manage a team that developed tools for traders, in excel. We had excel sheets that loaded 20+ custom addins, most related to financial libraries. The excels would compute prices of financial products in real time, and send those prices live on multiple platforms. Those prices were executable, meaning other clients and banks could trade live in a click.

This was (probably still is for sken banks) the norm 10 years ago. Most banks were running their whole trading desks out of excel.