Diesel can go everywhere, of course. My point is centred around the idea that moving freight by electric traction where possible is better than by diesel, in principle. If you disagree on that, then so be it.
The question shouldn't be how much freight is moving exclusively over 3rd rail routes, but how much freight spends a significant portion of its journey on 3rd rail routes, and the answer is a lot. The 92s are obviously ideally suited to routes with a combination of 3rd rail and overhead electric. Routes that are only partly electrified can be worked with a loco swap, if the will is there.
Now I know that you'll probably say that swapping locos en route wastes time and creates expense and doesn't add up economically. But, GBRf have obviously decided it's worth it on the china clay trains for example. So, to me that signals that it's not necessarily crazy, and maybe if other operators had access to (potentially refurbed) 92s, or even if more 92s were available to GBRf, then perhaps we'd see more widespread use of them than you might think. I do't know. Anyway, hence the comments about DB's mothballed ones, and the possible benefits of encouraging them to make them available for use.
I live next to the south London line which as I'm sure you're aware is one of the main freight routes across London. So aside from fossil fuel / climate change concerns, I can't help thinking that air quality might benefit from a higher proportion of the many freight trains that pass every day being hauled under electric power. 6 or 7 years ago, some were, under DB control. Now, none are (except the China Clay). Why's that? Apparently because DB have decided they are too much hassle (admittedly, transfer of some services to HS1 is also part of the story). But we are now watching with interest as GBRf, perhaps, prove that with the right modifications they can prove reliable and feasible.