why would you need to have so much SP just to have that much weight of voting influence?

The reason that influence is based on the amount of SP held is explained in the Steem whitepaper.

You can calculate how much an upvote is worth based on current conditions using this calculator: https://www.steemnow.com/upvotecalc.html

What good of an investment it would be if you can't sell and profit from it when the price is ripe ?

There are multiple ways to handle this. One would be to have some liquid STEEM ready to sell, while keeping the rest powered up. Otherwise, long-term holding is an option too.

How is this community different from the blue pill holders which empowers those who have more as well?

I'm not familiar with that protocol, but the Steem blockchain is designed to empower those who hold more SP tokens more than those with less.

Would the reward pool be refilled during the HF20 like it did during the HF19 ??

There are no changes to the rewards pool planned for HF20. You can read all of the changes here.