You are right "UMNO seems to be losing their grip in political power." Why would they to try the GRC system when they are losing power? UMNO would end up losing more seats in one go!
PAP did it when they were in the ascendancy, not when they are losing. Now that they might lose some GRC contests, they are thinking of reverting to SMC seats. The pappies are cowards! Single seats might lose. Now GRCs may also lose, so convert back to SMC and lose one at a time, instead of the whole lot!