I think you're missing the point of the trial, and the bigger picture. It's not about proving the reliability of 230s or whatever, it's about finding technologies, and trialling them in a real-world situation, that will contribute towards the aim of decarbonisation across the national (and global) rail network.
There is a place for electrification with OLE, but for many small branch lines (like the sort of lines that GWR operates, e.g. Greenford, St.Ives), there often isn't a business case for electrifying with OLE. At some point we will have to give up our reliance on using fossil fuels, so there is a desperate need to find a cost-effective alternative for those branches where electrification with OLE doesn't add up - if we don't, those branches are possibly at risk of closure.
That's what the trial on the Greenford branch is all about: it's a trial of technology, not a trial of a particular traction type. If it's successful, it doesn't necessarily follow that more 230s will be built from all the old D78 stock languishing in a field (although if that's a viable option, then why not?). But the technology could be licensed for use by other manufacturers in new-build stock.
Yes, it seems odd to keep a 16x sitting around at West Ealing, but if the 230 fails, it'll need to be rescued somehow, given that there is currently only one 230 proposed for running on the main line. Bringing a 16x from elsewhere is likely to cause unplanned disruption (given the lack of spare units at present) than planning to keep one available at West Ealing for rescue purposes. There might be some short-term pain as a result, but the push towards decarbonisation is probably the railway's biggest priority right now, which is why a lot of resource is being made available for the Greenford trial.
But as I said in a previous post, the initial plan is to keep a 16x at West Ealing - nothing stops that changing in future, releasing the 16x for use elsewhere.