I wonder how anybody can sell Mars with a 550 Earth days long trip...

If rockets could only reach Mars in 2 months at the most, plenty of things suddenly happen:
Shorter trips mean less time to test them beforehand, more testing, more time to change, to adapt according to the tests.
Shorter trips mean leaving Earth later, coming back earlier, more freedom in Mars orbit to choose the time for exploration, for a return.
Shorter trips mean less exposure to solar radiation, weightlessness, less corrosion to the equipments, less boredom, more attention from the public, faster return from investments.
Let's wait until they're accessible.