Garages sometimes diagnose a faulty ECU, they are nearly always wrong. They also often see "something something crank sensor" in the diagnostics and replace the sensor without a second thought or even a brief glance at the diagnosis instructions in the workshop manual for the part.
If the crank sensor was faulty the engine wouldn't run, this is actually consistent with your description in the OP. Perhaps they did confirm the wiring harness and connectors were all ok then they replaced the part.
Now it "doesn't hold revs", so the ECU is faulty. Not enough information here are all, you've paid for it so ask them for the diagnosis before agreeing to splurge on an ECU.
This is entirely speculation on my part but I think they don't have a clue what is wrong & the ECU is reporting no codes, so they just point at the magic computer box they don't understand.
Got to say that is my experience nowadays, they aren't mechanics any more, rather parts fitters.
Had an issue with an Audi once, garages couldn't diagnose the issue, amongst the faults were hazards coming on at random (whilst driving, whilst parked up and locked etc), took it to an auto electrician who just ordered a replacement hazard light switch ( at a cost of £28) which didn't fix the issue and they refused to send the part back, so I had to pay for a part which hadn't fixed the issue because they couldn't diagnose it properly.
The actual fault turned out to be water getting in to one of the ECU connectors due to a previous mechanic catching the rubber seal and it had rolled up slightly inside the connector.
There is an auto electrician local to me that REALLY knows his stuff, he is no good though, he is renowned around all of the local garages, he is a bit eccentric, very very disorganised, constantly late, likes to be thorough to the point his work is just slow. and frequently doesn't turn up to a booked appointment etc.
