I think they were just rationally optimistic. Designing a plane that met their other design objectives (fuel efficiency, cost, time-to-deliver, frame scalability, etc) while simultaneously meeting the US airline scope requirements was likely deemed to be impractical.
I say this because we're talking about 6 tons of excess weight here. The engines themselves are combined nearly 2 tons heavier than the original E-Jet family. The wing is 15 feet longer. A longer wing carrying heavier engines means a much heavier wing. Some of this is balanced by improved fuel efficiency (= less fuel required to fly the same distance = less weight for fuel) but these are major structural changes that require weight.
Taking the E175-E2 and tying to slim it down to a scope-compliant 76-seater will require either (a) a major redesign to optimize around this size, (b) tighter cabins to squeeze 76 seats into a smaller airframe than the current E170s, (c) de-rating that limits range when full of passengers (or some combination of a, b, and c).
I was hoping someone in this thread would address the extent of the impractical nature of the E2.
What would EMB have done differently if they knew scope would *never* change and had to build a scope compliant airplane?
Wiki ( https://en.wikipedia.org/wiki/Embraer_E-Jet_E2_family
Built on the first generation E-Jet, its wing is redesigned, and it introduces new pylons, landing gear, horizontal stabilizers, cabin, cabin air system, air cycle machine, bleed air system, and a new fly-by-wire system. The switch to a composite wing was not yet justified economically for a similar shape, the less draggy flaps are single-slotted instead of the more complex double-slotted on the E1, and the engine pylon is shorter. The raised, 11:1 aspect ratio gull-wing partially accommodate the 2.01m (79 in) diameter GTF, larger than the CF34 by 66 cm (26 in), while the trailing arm landing gear is taller for 23–25 cm (9.1–9.8 in) higher door sills, giving a 5 cm (2.0 in) lower nacelles than the E1.
GE Aviation, Pratt & Whitney, and Rolls-Royce were all possible engine suppliers. In January 2013, Embraer selected the Pratt & Whitney PW1000G Geared Turbofan engine as the exclusive powerplant.
What if they decided to do the simplest possible engine upgrade, was there no suitable powerplant available? We can see "the 2.01m (79 in) diameter GTF (is) larger than the CF34 by 66 cm (26 in)". Seems that's where they lost any chance of staying in scope.