Full agreement to your points here...which is why, when I look at the density and travel patterns and opportunities for further growth...^^ I agree. But generally, if you'd look at transport patterns, I'm sure that you'd find a lot of people from the fringe suburbs do commute to town. So I'm sure that with careful planning, you'd be able to plot the line's alignment along the higher density areas.
You did mention transit oriented developments. That is perhaps something that can be explored further. Like how a bus interchange, LRT/KTM Komuter station and a shopping mall (perhaps even inclusive of connected residential units) can co-exist on a single plot of land. That should boost the pull factor for the hub as well.
Rather than see KTM as a competitor, I think it would make sense to see KTM as a supplementor. Even a relief perhaps. The Subang-KL Sentral part does parallel the KJ Line somewhat, but I think it is located further away than the higher density catchments that the KJ Line tends to serve. Unless some efficient feeder bus system can be arranged, I don't think that KTM could be a viable relief or as you say, competitor in the manner.
Given the government's pace at implementing things, I wouldn't be surprised if we remain with 2 LRT lines in time to come.
I see PJ and Cheras as places where public transport is absolutely vital - along with the northern parts of KL - Kepong, Jinjang, Selayang, Wangsa Maju, etc.
Puchong travel patterns are directly towards KL...so an LRT line (or extension of the monorail with higher capacity) directly from KL Sentral along Jalan Klang Lama and Jalan Puchong would be a guaranteed success.
As for the Komuter, you are correct that Komuter cannot exactly operate as a "relief line" under current operations...but on the other hand, if KTM can improve their operations by creating more shuttle services and fast train services...then things will be a lot better.
Id love to see the LRT lines extended to meet the KTM Komuter lines for the sake of network integration. For example, a person coming from Klang or Shah Alam should have the option of a fast, limited stop train to KL Sentral or to switch to the LRT at Subang Jaya. A person from Klang intending to travel to Bandar Tun Razak should be able to interchange with the Ampang LRT at Jalan Templer...and a person coming in from Sg. Buloh or Rawang should have the option of a fast train to KL or the option to switch to the Ampang LRT at Kepong Sentral or new monorail line at Segambut.