If 18 months to switch is too ambitious, you are doing things wrong.
Why switch at all? You're promoting an ideology to a technical problem / solution. What benefit does your approach bring?
If you are relying on 20 million+ lines of spreadsheets, you are doing things wrong.
Again you're simply clueless as to how things works. Not all spreadsheets are simple datasets. There's a reason they were used in the first place, and one of those is that IT people for all their genius in administrating absolutely suck at doing end user's actual work. Virtually all attempts to replace complex large spreadsheets with something "better" fails.
20 million cell spreadsheets are NOT common for financial departments, or businesses in general.
Saying something repeatedly doesn't make it true.
That many cells being used in a spreadsheet can, and will, cause performance issues, as well as a slew of other problems, with Excel.
No other problems. Just performance issues. The cost of having a spreadsheet literally take 15-30min to calculate through it's cells is a minor price, one that can be paid during a lunchbreak. There's a reason Excel allows you to pause all calculation until you manually recalculate.
Everything that a spreadsheet holds (in terms of raw data) can be stored in a database from which you can easily export just the data you want to play with in Excel (or other spreadsheet software).
Congrats on demonstrating you didn't even attempt to understand the fundamental problem. The issue is not how to *store* data. That was never in question. You've mentioned databases, now come up with the 6-7 other applications you need to add to that database to replicate the functionality that you haven't bothered to understand.
Also exporting anything that sizable from a database to an excel sheet takes far longer than simply working in the excel sheet itself. Congrats, you broke the user's workflow, made it slower, didn't try and understand what was being done, and all for what? Because you think you know the answer without asking a question?
Let me make one thing clear, please don't ever work on anything without a project manager between you and your customer. Your complete lack of thought as to what is going on or what is trying to be achieved makes you the kind of IT person who gives the entire field a bad reputation. Do you work for Oracle by chance?