I am not going to refute the facts you have pointed out regarding Bangalore.
The 11302 Udyan is obnoxiously slow (just as 11013 LTT-CBE, in the opposite direction) and you can't pinpoint the issue to one specific zone.
No...
more... explanation was given for 16339/40 or 12649/50 rerouting which were blatant robberies from Bangalore passengers. Bangalore deserves a dedicated daily SF to both Mumbai and another to Delhi (via the shortest route) as a bare minimum.
Chalukya, they extended because it also solved the issue of Bangalore connection for Pondicherry and Tirunelveli back in the time, and only one additional rake was needed. Same was done for LTT-CBE further back, but now it's back to running with 3 rakes.
Some well-patronized trains from other zones (Sanghamitra, Seshadri for example) were extended to Bangalore as well.
As far as my rationale regarding Tiruchendur extension for this one, this is purely keeping in mind the intra-state passengers in Tamil Nadu (there is only one Chendur Exp with reserved coaches which is an overnight train) who would benefit.
A good number of passengers deboard in the DG-TEN stretch (at least half the train gets emptied at MDU due to the General Quota) and they are not replaced as the TPJ-TVC Intercity is operating right next to it in both directions.
It's not a case of taking a train bursting in capacity and burdening it further. This would be the equivalent of extending Hampi/Gol Gumbaz/MYS-KCVL/MYS-KCG Exp to Chamarajnagar (which has a lone crawler train to TPTY beyond MYS while most of the crowd for the trains I've mentioned for example here, empty out at Bangalore itself).
As I've said before, those from Tiruchendur (not a huge number if you consider TEN, VPT, MDU, DG stations all constitute General quota) who want to travel to Mumbai/Bangalore, they would not have much problems boarding the train at Tirunelveli in any case.
SR is unbothered to fulfill the bare minimum, that is restoring its crucial trains of the MG era (those running entirely within its limits, like the CBE-RMM Exp), it'd be a long time before they would get to the point of exploring the routes you've mentioned.
