For the 15 SP delegation, of course you can not set it to 0 as it's the Steem Power that gives you the bandwidth necessary to perform actions as you said. But 15 can be reduced, and multiplied by 900 000 accounts that has it, that begins to amount to something, even if you just reduce it by 5.
For the communities not reaching outside of the Steemit environment, that's totally true. But maybe the fact that communities could more easily receive massive delegations from Steemit Inc. could incentize developers to make apps that reach outside the narrow Steemit area. But developing projects (and putting time in it), with no guarantee it will attract users (in fact, the only guarantee you have is to be almost certain to have lost a lot of time). With a delegation from Steemit, you have a chance to attract people, to "pay" developers with upvotes, etc.
For improving lives, OK they improved life but it's the social networking in general that allowed that. The free masons (joke but true) are doing it since a long time, Facebook, Twitter and Instagram didn't invent anything, and Steemit does exactly the same as it is a social network. Steem can bring much more, but for now, it is reduced mainly to Steemit. And for people coming to make a buck from sh$t posting, I think a lot of these accounts are in the statistics, as Inactive. Just try to do so, you will see you will make no money (some upvotes from people with no SP) and nobody with enough Steem Power to give a real upvote will never upvote a sh$t post.
In fact a lot of them are so skittish about using their Steem Power to upvote others than themselves - or delegating to bots - that they won't even upvote quality posts. Sorry for the rant here, but that's an impression I have in my journey (of course they are exceptions), but the fact that "whales" (orcas and dolphins included) upvote only people of their kind (or are unintentionnaly circle voting) are at least as much responsible of the poor quality content here than the third world spammers trying to make a few cents.
For your conclusion, I agree at a 100 %. Steem has so much potential, we need to take action and make it as we want.