They could give better transport without the Olympics.
They could, but frankly its unlikely that that would happen in this day and age, especially under the last government who thought up most of the plans (this one seems to be leaning in a better direction from what I can tell, however anyone can be forgiven for not knowing what they are planning). Without the Olympics there would be no reason and no incentive to expand/improve the links, just as there is no reason right now for transport to be improved in Barking, despite such plans being on the table.
Why should the country spend so much on it
They claim the investment pays off but I doubt it when everything is taken into consideration like security costs and the disruption to ordinary lives.
Well, at least its on time and less over budget than it could have been. As for disruption, if that was taken as a reason not to do something, I don't think much would ever get done. Frankly arguing over whether it is worth it or not at this stage is pointless, as we don't even know how well its going to go. Heaven forbid, that extra security may actually be needed you never know.