That's not entirely true...

If let's say the minimum period were 5 days and for locking up SP for that minimum you receive no interest, then there's more interest to share for the rest of users.

If for 4 weeks it's, let's say 0.5%, then again it's less than currently 2% APR, so more for those who stake longer.

This way we can get to something like 5%? for locking stake to 6 months - 1 year?

These are not by all means exact calculations, nor can we have that at this stage.

By all probabilities this may be a dynamic interest rate, which will fluctuate based on how many people choose different staking plans. That's if we want to keep interest at 10% from total inflation, and I don't think that's under discussion.