I wouldn't say a "no brainer" means "there is absolutely nothing preventing it from happening". But eliminating diesel running under wires seems a fairly obvious objective to work towards as an ultimate goal.
Eliminating Diesel from under the wires isn't a no-brainer because there are lots of possible ways to approach the problem, all of which have different trade-offs and none of which is clearly ahead of the others.
Let's start with electrifying routes where DMUs currently operate under the wires, and convert those to EMU. Great, but now we've got different DMU services running under the wires we've just put up! Are we going to electrify 100% of the network, of course not! So are procuring Bi-Modes that still use diesel, just not under the wires, or BEMUs - but for both we need to consider required ranges between fuelling/charging, etc... Then there's the question of where to start, who's funding it, etc...
Compare this to, say, wiring the Poplars at Acton Bank. The problem was fairly straightforward - Elizabeth Line trains would lose access to OOC depot from the West. They therefore needed an alternative route to a depot. The potential solutions:
- Rely entirely on Reading/Maidenhead - not enough capacity for additional trains, and no room or time to expand them. Also limited justification to expand them for a limited time.
- Fit class 345s with traction batteries - expensive, retrofitting would require additional units out of service, long lead time to fit. Although would then be useful going forward any time the power goes off.
- Dead Haul - for the volume of moves required, would cost a vast amount to hire sufficient locos and drivers and not enough capacity to make the attaching and detaching movements at either end.
- Electrification - it's a short section between 2 electrified lines that then allows the 345s to run under their own power to get from one end of the Core to the other. The electric link would be usable by a wide range of potential traffic in the future, and could be delivered quicker than any fleet retrofit with minimal disruption to service traffic.
When to do it was obvious - it needed to be delivered before the OOC Blockade. The project causing the disruption were paying for it.
That's about the most straightforward project I can think of bar repairing Infrastructure signed OOU.
The next nearest to a no-brainer IMO is Didcot-Oxford Electrification, as there's already the stock, services and power supply ready for it. But it needs to wait until the trackwork is finished.