If you were an activist, then you'd at least have been an activist for a few years.
You assume that by being an activist you're automatically doing something useful. That's not self-evident to me.
That seems like a different issue. With earning to give, you take a while to start doing anything. With activism, you do whatever you're doing immediately. How useful that is, or even if it's useful at all, is a separate question.
When I talk to people about earning to give, it's common to hear worries about "backsliding". Yes, you say you're going to go make a lot of money and donate it, but once you're surrounded by rich coworkers spending heavily on cars, clothes, and nights out, will you follow through? Working at a greedy company in a selfishness-promoting culture you could easily become corrupted and lose initial values and motivation.
First off, this is a totally reasonable concern. People do change, and we are pulled towards thinking like the people around us. I see two main ways of working against this:
One implication of the "won't you drift away" objection, however, is often that if instead of going into earning to give you become an activist then you'll remain true to your values. I'm not so sure about this: many people who are really into activism and radical change in their 20s have become much less ambitious and idealistic by their 30s. You can call it "burning out" or "selling out" but decreasing idealism with age is very common. This doesn't mean people earning to give don't have to worry about losing their motivation—in fact it points the opposite way—but this isn't a danger unique to the "go work at something lucrative" approach. Trying honestly to do the most good possible is far from the default in our society, and wherever you are there's going to be pressure to do the easy thing, the normal thing, and stop putting so much effort into altruism.