You need to give them a tool that's as easy to use as a spreadsheet, and yet stores data in a relational database. There are plenty of spreadsheet-database hybrids in the market.
Even though Excel is proprietary too, it's ubiquitous enough that people don't have to worry about it.
And then, if somebody makes a change in the database, a trigger will update the spreadsheet
Such a two-way binding makes it possible to continue relying on spreadsheets for UX, all the while the data is not locked in there and we can also have other processes handling the data (a web app, some cron job, etc)
Maybe market it as an API for excel or something
See here to understand what you are missing out by not using a database: https://visualdb.com/comparison/#integrity