There has to be a maximum block size limit for bitcoin nodes to work.  The limit is not just a program variable needed for block chain consensus, it has real world implications in terms of storage, processing and bandwidth resources.  If a node doesn't have sufficient resources it will not be able to work as a properly functioning node. These resources have to be provisioned and managed by node operators who have to plan in advance to acquire the needed resources.  That is the reason for BIP 101 having a schedule for changes to the limits. A dynamic algorithm can not magically instantiate the needed resources.

The schedule in BIP 101 is based on technology forecasting.  Like all forecasting, technology forecasting is inaccurate.  If this schedule proves to be grossly in error then a new BIP can always be generated some years downstream,  allowing for any needed "mid-course" corrections.