I am not saying that they should have built the entire line in one go. They could have built the at grade portion underground 10 years later.
To your second point, yes, that's what exactly happens in every city in the world. Not everything can be built in the city in one go. Some places have to wait for longer. We didn't build Lines 1, 2, 3, 4, 5 and 6 at the same time. Some places had to wait because some other places were a priority. It's better to make something more useful with a wait of 10 years than make something that's not efficient and have it run for 100 years.