Well, anybody who imagines that 4e will stay evergreen and just get revised via online updates has their head in the clouds. Of course there will be a 5e; it's just a matter of time.
I'm not so certain. We know that developing a new edition involves a pretty hefty upfront investment in time and money. Estimates in this thread are that it takes +/- 3 years of in-house development to get a new edition off the ground. I presume a hypothetical 5E would also need online tools available soon or immediately after launch. That's an additional upfront cost.
I can imagine a scenario where WotC tries to avoid these costs by doing what it's already done: releasing products like Essentials that take less time (and presumably cost) to develop.