They should freeze development on this season except for those things that will aid them for next season. There literally is no point spending even a minute on improving the car for this season only. They've already easily won both WDC and WCC, it's done. 2020 is going to be a massive challenge.
2020 regs are similar to this year. I'd expect 2020 cars to be natural evolutions of their predecessors. Unless Merc's 2020 car is radically different from 2019, given the lead they have in both championships, they could have used the end of this season to test new parts designed for the 2020 car.