If you curtail the service back to Middlesbrough you start running into the same issues that caused the service to be extended in the first place because, as yet, nothing has really changed on the ground. If you terminate at Middlesbrough with a long lay over you have the issue of where you put the TPE set. It can't fit into the siding to the west of the station unless it's only going to be a 3-car 185. The main carriage sidings to the east are a pain to access (upon terminating in platform 2 you have to change end, shunt back out of the station to the west, change end again, then traverse P1 before getting into the siding). Certainly not impossible by any stretch but a pain. Also often they can be full of Northern units (at least the bit that's cleared for use, the whole length isn't in service for anything other than Network Rail tampers and the like). Otherwise it's going to have to go down to Guisborough Junction (on the way to Nunthorpe). So if it's a short turn around say ten/twenty minutes or so (which is what they were before the Redcar extension) that's probably workable. The branch isn't that busy, but the current layovers of around 50 minutes are going to be impossible to fit at Guisborough junction. But the shorter turnarounds have implication for service reliability, which was the driver for extending the service in the first place.
So that means the easiest way to get out of the way is to go to Redcar and then shunt out of the station into the loop there and then sit in the loop. That keeps it nicely out of the way but it can still rapidly cause chaos when TPE are late and don't have time to actually go to the loop and come back again. I've experienced on multiple occasions Northern services picking up delays due to TPE being in the way at Redcar. So then that makes Saltburn the logical place to go get TPE out of the way to have a turn around which is long enough for service reliability but without gumming up Northern (since they extension to Northern it's been a long while since TPE have caused my trains to be delayed).
Now there are obvious political benefits to be sure. Hi Viz Houchen has been making hay as has the Redcar MP Jacob Young when the service was extended to Saltburn (which was odd as that's not in his constituency). The connectivity is also nice to have as well. I've taken advantage of it myself on a few occasions. I'm not convinced it's quite as poorly used as some have suggested above but it definitely seems time dependent (trains during the day seem to load a bit better than say the last train or two of the night) and I certainly won't deny that often a 3 car 185 will be carrying only a handful of passengers. But I've also seen mid-afternoon trains loading slightly more healthily similarly on weekends as well the morning trains down to York seem to do a bit better.
Broadly speaking I'm not sure it's quite as simple as saying "start terminating at Middlesbrough again". There were solid operational reasons for extending beyond Middlesbrough it certainly wasn't just for the sake of politicians egos.
Oh great, that's a service I use not infrequently to get back from York to Middlesbrough/Redcar. Now I'll probably end up having to hope that conductors can't be bothered to issue a change of route excess when going via Darlington otherwise I'll be stung for £5.65 on top of the £21.30 that a return not via Darlington already costs. Fantastic
I wonder if this is crewing issue? Easier to find a conductor that signs the road to Darlington rather than to Saltburn perhaps? I've noticed that that one can be prone to cancellation or delays waiting for a conductor (there seems to be a conductor swap at York).
I've been thinking for a while that there clearly should be a call at Marske to take advantage of that dead time...