We constantly make purchasing decisions based on our (hopefully) reasonable expectations of the future behavior of the seller- particularly including matters like continued support for the product.
"Forever," is not a reasonable expectation, in my opinion.
This is not to say that I want or expect them to shut off the power the day the first 5e book hits the shelves. That's not necessary from a business standpoint, and would be a horrible PR move. But, eventually it won't pay to keep the support running.
The DDI is software. It will have been written assuming some infrastructure underlying it (hardware, operating systems, browsers, and so on). As time goes on, those infrastructure bits become obsolete, and slip into non-supported states themselves. So, maintenance of a system requires either occasional porting to new infrastructure, or ever-increasing fees to try to keep a legacy system alive*. The population of subscribers is going to decrease, and eventually drop to a level where the support isn't an economically sound proposition.
Realistically, then, it isn't a question of if they'll end support, but when they'll end support.
Well, I agree, to a point. However, it would definitely sway my decision as to whether to subscribe to a 5e subscription or not.
And, I am sure there are folks out there with a wide range of expectations.
I, personally, am not a "one game, for all time" kind of gamer. While I like my campaigns to be long, that's in terms of being a couple or a few years, but not decades. If I were playing 4e, and they gave me a year or two to wrap up my game, I'd find that eminently reasonable.
But then, currently, I run Classic Deadlands, which is only in print in PDF, and is getting no further support. I play in a Star Wars Saga Edition game, which isn't supported even that well. Playing without official support is what I normally do anyhow.
*This is ignoring the security issues legacy systems present, and those can kill a system faster than the compatibility issues do.