Sure, but if you make the last events more important by 'adding a playoff style event' - you're effectively minimizing the impact on all the previous events. In essence, all you are doing is shifting your viewers from one event to another. If you're doubling the event - the people who in previous years perhaps didn't watch the last race because the WDC was already a 'done deal' might skip on the second last event now if they know it's the last one that will ultimately count and all is decided. In this scenario, all you've done is shift the focus and upsetting the other countries event by limiting their appeal.
Not that I believe that the majority of serious racing enthusiasts watch the sport this way. If numbers are droping due to a boring fest - they should be thinking up solutions to make the races more interesting / more dramatic. They could start for example by taking a good look at the tyres (and what they want to achieve with them), the points system (perhaps decrease the gap between win and the rest of the field to narrow the gap) and as they're already doing, find a way to create a more 'level playing field' by setting up rules to maintain a cost limit for all teams that works.
It would also be good if we had tracks that distinguish itself more from the rest - so that teams might have an advantage on some tracks, but less on others, so that we would have more teams fighting for wins, and not a singular team racking up win after win. This would create far more excitement and a larger audience than adding a double-points system for the last event.