"stupid" is the wrong word - there remains a distinct, albeit niche gap in the market for RA5 locomotives.
Not sure there is actually anywhere on the network that a class 66 cannot get to, albeit with "paperwork". Possibly the Oban line from Crianlarich and possibly a secondary route down in Kent.
I was brought up on the Cambrian in the 1970s and was under the impression it was Sulzer type 2s and class 40s then, although 37s were actually cleared but rarely ever used as they weren't LMR locos. Then a WR class 47/0 with slow speed appeared at Machynlleth one Friday evening. It was for mole plough work as the S&T were burying their cables. The 47s got dispensation to work as far as Dovey Jn.
Things changed in the early 1980s. Class 20s appeared on the Aberystwyth-Stanlow tanks, 31s replaced 35s and then 37s appeared.
Much later, the heaviest RA locos (class 67) appeared during the EWS era as well as class 66s. Sadly though, all the work that went into clearing the black five 4-6-0s for the Cambrian Coast line for steam work, came to nothing. ERTMS replaced RETB. I believe they were going to add a false floor to the tender to lighten the RA a little and put the ERTMS equipment under it. I believe the costs of fitting all the equipment and getting it certified were not acceptable.