One problem with the OP's suggestion is that the Met Line trains are S8 (i.e. 8 cars); only the (current) Met Line platforms are set up for these (and, in the tunnel sections, long enough). The Circle east side stations (from Paddington to High Street Ken) have I think some of shortest platforms on the sub-surface lines - pre S stock they were strictly 6 cars max; and S7 trains can only work with a lot of SDO.
But that having been said, I do think that when the sub-surface lines were reorganised when the S stock came in, to withdraw the round and round circle, and increase the number of trains to Hammersmith, some tricks were missed: I would have looked at whether S8s could work east to Upminster (I think most of the platforms should be just about long enough), and what other options there might be in connecting up the various sides of the Circle, and all the branches off it (e.g. Hammersmith-Aldgate-Wimbledon?), being prepared to cast aside all the traditional line assignments/pairings - going back to how the LPTB to have all the sub-surface lines (both Met and District - the Circle, H&C and East London weren't distinguished back then) as one group, in green on the Tube map.