We don't have transaction fees to control how many transactions are made, we have rate-limitations for that reason.
Well, the main user group that is impacted is new users. I don't know if that is a good thing.
If somebody wants to use the platform more, they should either earn or buy more SP.
It's a valid idea, and one of the options I presented - but I don't think it was the original design. One of the main goals is to onboard new users to the platform. Things that may make people run the other way should be considered carefully.
Of course new users should be given enough SP to use the platform without too much disturbance. And they could be offered a chance to buy SP when they create an account so they can be sure that no problems will come in the near future.