If you want to be one of the top ranked players, then you'll need to either:

a) be a whale, or
b) be able to raid assist for hours per day

The latter doesn't mean focusing on the game for hours per day. It does mean being at a computer or near your phone while doing something else, and then checking back every couple of minutes to join another raid.

Regardless, if you want to be one of the top players, you'll also need to be able to only infrequently go over 10 hours without logging in and using your action points and so forth. If you can log in briefly in the morning before school or work, send the next batch of girls on purification, burn through your action points and arena banners, and then log off, that makes an enormous difference.

That doesn't mean that you can't play at all if you only play once per day. But it does mean that you'll never be one of the top players. Which is fine if you don't feel the need to be competitive.

The game isn't really dominated by whales. I've probably spent about $50 in total since the game launched, and was recently the #1 player on my server in total squad power. I've also had several weeks where I went undefeated in arena while always challenging the higher ranked player for 1.5x rating gains.