If you require QuickBooks Pro 2013, then you require a newish windows / office, which require new hardware + new licenses...
Ok, how about another version of QuickBooks? Every version from 2009 onwards either doesn't run or runs like crap.
I don't know of any useful userland linux program that hasn't been ported to windows...
Which is irrelevant because the suggested direction is Windows to Linux, not Linux to Windows. What is needed is Windows apps ported to Linux.
You'd never switch to linux to increase productivity by upgrading your tools...
You are thinking like a techy, not a manager. Productivity is a primary concern.
1) It's easier to mantain than widnows, (for the little guys), and that reduces support costs.
Having supported both in variously sized companies, I can safely say this is a false statement. In fact, the primary reason for maintenance was people doing non-business things(surfing porn, etc) on the business computers.
2)It's more secure than for the user (not because of the os, but because dancing pigs won't be able to dance on linux). That also reduces support costs.
I am not sure what you mean by "dancing pigs". If, by "dancing pigs", you mean users, then you are also talking about increased salary costs because higher quality workers cost more. If you mean downloadable games, apps, etc., then you are talking about people doing non-business things on a business computer.
3) It provies longer life to your hardware, saving you money.
Except hardware is a depreciable capital cost and the only time one needs to upgrade is when one needs better hardware for an application or just preference, which would still result in a new purchase.
4)Its free and that saves you money.
If you haven't noticed, it is often times cheaper to purchase a new computer with the latest version of Windows than to purchase one with no O/S or to build a system.
The mayor obstacle for typical environments that don't require special sofware are the huge amount of VBA excell files. If your company doesn't use them much then with some windows boxes kept for those special case you'd befine.
Remember what used to be said about IBM? Now, it is "No one ever got fired for going with MS Windows on the desktop." Especially, with the popularity of MS Office. Another big reasons is Outlook and Exchange.
You are looking at this from a tech angle and are missing a lot of things management and business owners look at, including lost productivity, retraining costs, and all the other switch-over costs.