SNI works with anyone on Windows > XP, and any mobile / Apple / Linux OS you'll find visiting your site. How many IE6/XP hits does your site get per month?
The bigger problem right now is Android 2.x. But the lifetime of phones is only about 2 years and the market-share of Android 2.x has dropped below 20%.
And it really depends on the audience. The market share of all IExplorer versions on my blog is less than 4%, out of which IExplorer 11 represents more than half of that.
Below 10% even[1]. Agreed on device lifetime. Unlike PCs that can stick it out much longer, give it another year and it'll be well below IE6 levels. Can't come soon enough.
[1] https://developer.android.com/about/dashboards/index.html
If SSL was truley as trivial as a check box, then we would see very wide adoption.
The "100 times as much" is not that impressive, if it merely goes from 6 cents to 6 dollars.
6 cents vs 6 dollars is the difference between a person in a low-income economy spending 0.1% vs 18% of their income on a website.
We need better infrastructure.
Cost. Today I host my website for 6 cents a month as a static page on Amazon S3.
To go https, I'd have to first acquire a certificate (lucky that can be free and with Let's Encrypt will be). Then I have to find someone to host that certificate. I can pay someone hundreds to thousands of dollars in setup and monthly fees.
Or, the cheapest option I've found is to get a $6/mo VPS as a frontend, put nginx on it, and put my cert there. The problem is that costs 100 times as much as what I pay now, and I have to maintain a server and it's not served from multiple redundant servers like my site is now.
Or I can use Amazon's free SNI support, making it so that older browsers can't see my site and I have no way of blocking people from using http or redirecting them.
There is currently no good, cheap option to do SSL only that is viewable by everyone.
That's why I haven't switched yet.