"Decades" is a very long time for a car. That bit of your request makes me reluctant to suggest anything that isn't a Honda or Toyota.
Seat comfort is highly subjective, partly because no two backs are quite alike. Spend as much time as you can in each contender before buying.
Nearly any current car will run on premium without damage. But if premium is recommended, as it is in most luxury cars, performance and fuel economy in city and suburban driving will suffer if you put in regular instead.
25 mpg in the city plus strong acceleration isn't an easy combo in a luxury car. This largely rules out those with V6s unless you're going to be very easy on the gas pedal--but then you won't have strong acceleration.
The Acura TSX, which will be well under your price limit, probably comes closest to meeting this set of criteria. If you can relax the mpg requirement, perhaps the Lexus IS or GS. (The hybrid of the latter might achieve the 25 mpg, but I'd be a little concerned about owning one for decades.) I'd like to suggest an E250 diesel, but while Mercedes reliability has improved recently it's not possible to say how a 2014 will fare when 10+ years old.