I'm not a big Machi Koro fan myself, but I was sold on the legacy aspect having never played one before, and my thoughts pretty much echo yours. My wife and I played through five games over the weekend and we cannot wait to play some more. It can still be a little luck based towards the end of a game with both of us just racing to see who can be first to get the amount of money needed to buy our last landmark and win, but the new additions and alterations to the rules every game make it a lot more interesting than vanilla MK, and I'm really enjoying it as a result. It may just be our game, but the nastiness of the purple cards seems to have been reduced significantly too, which my wife definitely prefers.
Regarding what happens once the ten games are over, apparently we should all end up with our own unique new base game to continue playing endlessly, and Tom Vasel said in his review that he prefers the game he has ended up with over the original. Whilst nice to have a still playable game afterwards, if I never play it again I will still have had enough value from it over the ten hours or so it takes to complete. I've spent more on tens of video games that I have barely touched at all!