I’ve reached 100% Fuel Efficiency and Tyre Wear, why would I build more Powertrains?

You’re absolutely right, and once you’ve reached 100% Fuel Efficiency and Tyre wear and Powertrain nodes will auto-complete, meaning you don’t need to waste your hard earned cash. If you decide to stay in a Championship where you’ve reached 100% in both, the next Seasons Tech Tree will not feature any Powertrain nodes.