MLS works fine in rain.
The FAA favoured GPS only after fighting for world leadership of the MLS design with an inferior system, then discovering that the costs of re-equipping was not justified by their requirement for Cat 3. Thus the already funded GPS was selected as being capable of achieving Cat 3 integrity in the future at lower cost.
MLS was demonstrated at 1 deg glidepath showing interference free operation, and for potential military / helicopter applications.
The LCY runway was initially limited to 1199m to comply with the ICAO code 2 runway requirements (1200m limit). This provided the necessary lateral obstacle free zones and distances to the water hazards in the event of lateral or longitudinal runway excursion. With evolving common sense it has been possible to lengthen some aspects the runway whilst maintaining obstacle clearance (height of parked aircraft tails) and providing alternative overrun protection.
Runway extension to the West is not easy due to the (real) approach obstacles, but to the East with the absence of the mythical East London River Crossing Bridge the approach could be adjusted. The 5.5deg slope is retained for noise reasons.