Why is there no “MFN lump sum”?

In case of a royalty percentage there is an ongoing obligation. In case of an ongoing obligation it makes sense to have a mechanism that may lead to decrease the royalty percentage when a new -lower- percentage is agreed upon with another licensee.

In case of a lump sum there is only a one time obligation: once the payment is made there is no longer a payment obligation and therefore no amount or percentage left to be lowered.