Aircraft today do look similar for reasons discussed but, of course, under the skin they are very different to their forebears in terms of materials, systems and technology. Even though e.g. the wings look similar, today's wings are hugely more efficient than those of earlier generations due to advances in computational fluid dynamics among other things. But even if there is a revolutiionary technology out there which will transform economics it is unlikely that any manufacturer will take the risk without some kind of government support - either direct or through the military. The B707 was, of course, a development of the military-sponsored KC135 and even the B747 design was part paid for by the US Goverment during the competition which ws eventually won by the C5A. The Blended Wing Body (BWB) seems to offer huge efficiencies but the risks involved (technical and pax acceptance) are enormous. If the military decided to adopt it for their next generation transport, it would ease the path into eventual commercial service. The Rutans are great free thinkers but, on the other hand, it was Burt Rutan who designed the Beech Starship!
Will there ever be another supersonic / transonic transport or will the Sonic Cruiser / 7E7 phenomenon always come into play i.e. airlines will always want the technology to reduce costs rather than increase speed?