You have to remember that an SEP Class Rating is just that! It gives pilots the privileges to fly any SEP Class aircraft with very simple familiarisation or differences training.
Hence keeping an SEP Class Rating valid on Microlight flying alone is unlikely to be acceptable to a competent authority - which is why the consolidated NPPL revalidation criteria for NPPL holders with Microlight and/or SLMG and/or SSEA Class Ratings allows them to complete their total flight time requirements in any of the classes for which they hold a rating, but they must fly at least one hour in each of such classes.
Which has proved perfectly safe; no-one is going to rent out an aircraft to a pilot without any recency in the class and any owner will probably fly rather more than 1 hour every 2 years in the aeroplane.
IAOPA(Europe) has proposed something similar in its NPA 2014/29(A) response (18 months ago....), but we are still waiting for EASA to release the Comment Response Document, which is long overdue.