If your dealing with European marketspace, or say Asian marketspace, or even South American or African, you should pick the best solution supported in the area and can continue to maximize business value.
That said, Thunderbird does not integrate with Exchange to share Calanders at the 'simplist one click level' consumers get with Outlook. Thunderbird / nor Libre Office can tie into Back Office for Project to integrate percentage results of completion in Work Breakdown Schedule, nor on the fly show current and project project costs based on the therom inputted during the Project development phase. OS X makes all those using the expensive and highly supported Apple products a great 'easy' product, but still it was Apple that broke down and begged MS to create Office for Mac so the Apple's could do the very same thing (intergration with Microsoft backend) I am talking about. And so on and so forth.
Every business can't be looked at as a niche, as every business is expected to perform the SAME legally required aspects, wether they make Linux Software or Microsoft Slate or pick up my trash outside and deposit it in a landfill. They all have the same basic structure, they have the same payroll methods (work per hour at given rate, state tax removed, federal taxes, etc. and so on), they have the same land / building permits concepts, the same demands for insurance needs, legal standing as a Corp/LLC/etc. and so on and so forth. The BUSINESS side of it, is always the SAME MODEL, SAME RULES, SAME responsibilites in the end (be it profit to a organization, stock holders, only one business owner, etc.). Now the nuianes (the individual tree brances, and leaves) is where your focusing on and yes are niche to the business in general terms, I am saying a forest is a forest is a forest. Water sun earth needed to make it grow, has four seasons, and so on, your wrapping into a more granual differentiating by the way your representing it.
As for the move to SaaS,, your generalizing 'computing needs' like your customized bookkeeping program into the same thing as the GROUPWARE (the point you keep very much overlooking) which it isn't. When you made your customized bookkeeping program, does it integrate into the Backoffice to generate automatic billing to customers by pulling the data from the SQL Sales server then tying it into the Microsoft Dynamics ERP to show "Real-time financial data"? Does it generate fiscal forecasting models that automatically tie into the Backoffice for all the Projects scheduled and being scheduled funding so that the instant decision on which Projects needs to be saddled and the cost outlay for doing that? Does it generate notification via a Calander add on instant Notice to Outlook via Exchange to the customer's Sales Rep (which is pulled from a seperate SQL server running CRM) to make sure the billing is paid on time? And so on and so forth.
See I am not talking A webpage, for a couple of people in a Mom and Pop store of Mom and Pop and that is about it. I am talking any level from a Dozen employees (SMB) to as much as 10,000 world wide locations is what Microsoft has made each 'tier' of the product lines over the past decades. So there is multiple solutions, which we all may not like, but really there isn't a 'competitor' to these solutions that provides the 'front office' these flexibilities THEY DEMAND.
Again, if they just need to surf websites, read some webbased email, or simple stuff, sure Linux MAY be a solution as much as Google is. But if your talking ANY of the real 'business of business' end I just mentioned (which is from SMB to Large Corp), then your dealing with the Microsoft solution.