The transport sector decarbonisation represents an essential part of the GHG emissions reduction strategy of the EU for the 2030 and 2050 horizon. The strategy presumes a far-reaching structural system change towards the marketing of advanced electrified powertrains and use of energy carriers such as electricity and hydrogen that are generated in a carbon-free way. The decade 2020-2030 is expected to set in motion the complex process of the necessary transformation of the power generation and transport systems which mandates the successful coordination of the interrelated market actors, consumers and policy makers. This paper presents a systems analysis that aims to assess what would happen if the decarbonisation strategy fails to develop, as anticipated during the decade 2020-2030. We refer to this timeframe as the "Lost Decade". To answer this question, our analysis is based on counterfactual scenarios quantified using the PRIMES energy systems model. The analysis concludes that the decade 2020-2030 is a critical point in this transformation. Limited progress in the coming decade would require the adoption of stringent policy measures beyond 2030 which, along with the pressure to reduce emissions in a short period are found to increase substantially the total costs of the system.