Why not start it at 5x without any SPS staking and then make SPS staking beneficial from there?
Many of us have staked all our SPS in Splinterlands and still haven't made it back to 10x (i.e. previous rewards) -- and now we're going to have to pull some of that to get anything over 1/10 rewards in Soulkeep.
Increasing the value of SPS is awesome -- but this could also kill Soulkeep before it has a chance to catch on (1/10 rewards are ridiculously low).
Even if part of the SPS staked for @splinterlands counted for Soulkeep, it will be a good start IMO.