As it is relatively hard and costly to create an account I was thinking running a bot that upvote content relative to the amount of votes acquired after 24h, regardless of the SP behind it.

I really don't understand how you are surprised that people who created a free account and got given 5$ worth of SP don't have as much influence over the platform as those investing their lifetime savings into it. At least they have bandwidth and they can post, comment and get paid doing so.

Almost nobody is currently buying steem to have more influence on the platform. The platform is not popular enough, there is not enough people watching the top rated content to warrant throwing money at it. People are mostly investing for the long term expected value of their SP, the platform need development work on many front. Steemit UX, 3rd party clients, 3rd party apps, content creator, curation, advanced tool, development of good practice, documentation, strengthening the steem blockchain. This takes money but also lots of time, trial and error to get right.

I understand that you feel like people with lot of steem are not responsible with their SP and that the SP is not evenly distributed.
Every whale has the responsibility to improve the platform in many ways, abstaining from voting if not sure about the long term outcome of their action is one of them. That might be one of the reason why NED, Dan and @Steemit don't vote. They are long term investors not normal users.