"Excel is a swiss knife type tool every accountant knows on a level you would not master in years."
Very true, but why do people assume that a general migration to Linux means EVERYTHING must be Linux?
In my own limited experience, most folk are happy with Linux for many things, and the few business-critical programs you really must have can often be run in a VM of Windows. Said VM can be minimal, have limited network access, and generally is a very secure way of doing things (given that a lot of smart malware avoids running in VMs to evade analysis).
Sure it is an extra training step for those users, but my 75 year old and largely computer-illiterate father was able to master VM use for a specific genealogy program. I'm sure your accountants, etc, would manage it fine if given a couple of minutes tuition and a cheat-sheet of things to remember.