It's probably worth remember what killed the 757 in the first place: The 737NG got too close to it's capabilities and cost a LOT less. The 737-800 had close to the range, and the -900 close to the passenger count of the 757, burned less fuel, and cost less to buy. While some operators needed that extra margin of range and/or passenger capabilities, it wasn't enough to keep the line cost effective - near the end the 757 was down to one/month - which drove the per aircraft overhead costs of keeping the line open through the roof (plus, the 757 was always a relatively expensive aircraft to build).
15 years ago, not many operators used 737s and A320s for long haul operations. But as point to point has grown, so have the number of relatively long haul 737/A320 operations. That extra margin of range and passenger count is now more desirable, and the 737 (and to a lesser extent the A320 series) can't provide it. Further, as others have pointed out, 6+ hour flights crammed into the back of a narrow body aircraft sucks - especially to those who've become accustomed to relative roominess of a wide-body.
A 'new' 757 would require a new wing, new engines, and new flight deck to be even remotely marketable, and you'd still be stuck with that (relatively) narrow 707 sized fuselage. If Boeing is going to dump that much money into a "757X", why not just bite the bullet and do a new, state of the air, Mid-Market Airplane (which, as I've noted before, I expect to be twin aisle)? A family of MMA, with seat counts from ~180-280, 5-6,000 mile range, and seat mile costs better than either the MAX or the NEO would relegate the narrow bodies back to short haul and dirt cheap charters where they belong. The biggest hurtle to a new MMA is the lack of a suitable engine - currently there is nothing in the 40k-50k thrust class...