.......... eventually it will be unnecessary to have any pre-engine
........

I dont think thats the case at all from filter 'experts'. You will
always need a 'choke' or 'last chance' filter to ensure protection.

A recirculation system reduces the particles 'offline' and therefore
virtually eliminate the 'particle challenge' to the racors. ......
meaning that the racors wont see particles; but, nowhere from any
filter expert will you find any statement of elimination. With a
recirculation system you may consider to decrease the 'size' of the
Racor and lessen the need for a parallel Racor installation (just a
hard piped bypass) ... but still must allow for the volumetric flow
(delta p) through the racor.