One of my last jobs was at an agency that handled the digital account for a very large airport in the UK. They get a lot of traffic, and quite a high percentage of users using legacy browsers, probably from businesses looking up flight info and trying to book stuff.
I can't remember how high it was, but let's say it was around 10%. We were told that we had to support all the way down to IE6. To do so, we raised our prices, almost constantly during that project, and they happily paid it. At that level, an agency can change thousands for legacy browser support, and the client will happily pay, because in their mind that 10% will spend far more money than what we can charge.
With that in mind, that's how I view browser specifications today. For clients with an existing website, I check what percentage of their users use legacy browsers, and work out with them whether the cost of supporting these users against the potential benefit. A lot of developers and agencies simply won't do this because they either can't or won't find a solution for legacy browsers.
Enterprise also means six-month sales cycles, finding salespeople with good Rolodexes, endless awful meetings needed to sync up five different departments, and a diminished ability to turn technical superiority into paying customers.
There's a ton of money there, but you need to build the right organization. Some companies will find it easier to sell to smaller businesses.
What business are you in? If you're going to support IE6 until 2019, which probably runs Javascript 200 times slower than IE10, your software is going to be very limited.
Once your competitions builds a better mousetrap, sends in their sales team then explains that all your customers need to do is install Chrome, it might get a little ugly.
We do a diagramming library. Try http://www.draw.io in IE 6 and 10, I suspect you won't notice the difference.
Fully supporting IE 6 doesn't constrain you at all with the modern browsers, we take full advantage of the recent stuff. We basically have 2 codebases wrapped up behind a single API.
That's assuming you didn't already tell them about the limitations of the environment they're choosing and explain stuff like ie compatibility modes that can let them move forward but still use all their cruddy old ie6 only enterprise apps, etc. My experience with this is you want them, cut features out if on those old browser versions and periodically remind them that they're holding themselves back. The scenario where they force vendor a to use ie6 and allow vendor b to use chrome IMO doesn't exist if you discuss this stuff up front.
It's a myth that IE 6 is devoid of functionality, we have no loss of functionality. The big problem is that it does everything differently and nothing is documented :).
1) Because they are Enterprises.
2) Because nobody wants to support those browsers.
If you think it's a declining market that'll be dead in 18 months, we have contracts to support IE 6 until 2019, not to mention 7 and 8.