I don't think there is any conclusion reached about what caused the sinking, but at that time I think i read an article explaining that nanggala should have underwent maintenance but being postponed and then the accident happened (I could be wrong tho).
Regarding the ToT scheme I think its more about marketing strategy, SK want to be involved more and thus some submarine will be build in SK and Indonesia will only assembling submarine locally at first, after several submarine being build by SK then Indonesia will be allowed to completely build the submarine locally. Meanwhile France will be offering straight up locally build submarine so they can enter Indonesian market and compete against SK. What makes SK offer is quite tempting for me is that Indonesia will be allowed to export it in the future (said the previous MoD), not sure if there will be any country that will be interested with that tho.
Rather than abandoning new submarine procurement, the govt decide to switch vendor instead there must be reason for that. Although it is still unclear if MoD really want to ditch the contract with SK, if he is chasing for quantity then who knows maybe there will be nagapasa batch 2 and scorpene, and maybe interim submarine.
Currently Indonesia only have 4 submarine, 1 of it is too old and it is better to retire it soon and that would leave Indonesia with only 3 submarine, the current target for submarine number is 8 and after that 12. We have serious issue about quantity and quality.
How many crews are actually ready to operate the submarines, like ready-ready, not just based on number of graduates but people who can actually take a submarine out to an operation or advanced exercises? After nanggala sinking, assuming there were some trainees along with the experienced submariners, there is definitely a shortage on the crew to run those submarines (as well as the interim ones to be received).
What makes sense is increasing capabilities of a shipyard by following a ToT scheme, the contract with SK was logical for this reason, it has delivered capabilities step by step and it was doable. However absorbing a whole submarine construction ToT is not quite an easy job, especially for a submarine type and construction methods PT. PAL haven't been familiar with, after Type-209's construction.
I think quality of a submarine force is rather defined by quality of the crew rather than the equipments. An inexperienced crew with a top-notch submarine (let's say Scorpene) may not be considered as strategical as a Type-209 operated by a crew who has been trained by submarines who have been familiar with similar platforms. I would say add +10 years to call Scorpene, after its delivery to the Navy, as a strategical submarine force asset.
Meanwhile Type-209s were mastered by Indonesian submariners, they knew similar-hull's almost all characteristics, got used to the CMS, do and don'ts of the submarine, capabilities, signatures by all means. I can detail this further with pumps and any other mechanical switch and valves and particularly weapons and electronics; it takes time to get known to a maritime platform, let it be a ship or submarine.
