One of the first things, but not the first. Actually being able to get to their destination, and getting there quickly, is more important than having a seat. Providing seats is a luxury we can only afford when we aren't subject to other major constraints.
On a major route corridor like Leith Walk, dwell times become a dominating factor on overall capacity and journey times. Buses set up so that everyone can have a seat (double deckers!) mean that it is harder for people to get on and off at each stop. Adding a second set of doors helps. The bottleneck can then shift to the stairs up and down, so you end up with a three door, two stair solution like the New Routemaster. With each iteration, you lose more and more seating capacity and rely instead on standing capacity. Trolley buses and trams take that to the next level.
Maybe the reason people are so desperate for a seat is that the buses take so long to get around the city? Maybe a solution with reduced dwell times can reduce journey times by enough to compensate for any loss in seating. Standing only becomes a real problem once people are onboard for more than some reasonable amount of time - e.g. 20 mins.