Why fuel security shaped the Revenge class

Were the R-class battleships intended to be obsolete second-line ships from the start?

No; their more conventional design reflected concern about oil supplies, not planned obsolescence.

The R-class battleships were built to a more conventional design than the preceding Queen Elizabeth class. That choice reflected anxiety about oil supply rather than an intention to produce obsolete second-line ships. They were meant to serve as battleships from the start. Fuel security could shape capital-ship design before combat experience exposed any later limitations.