There's also the financial matters to consider, a debut means a regular salary, as well as any outside work that might come along. I think it's been talked about before that there have been girls who wanted to join h!p, but spending years in KSS with only a possibility of debut wasn't a viable option for them (even more so if they don't live in Tokyo).
Yes, as you mentioned, there have been KSS that decided to quit the KSS due to financial reasons, because Up Front doesn't pay the transportation expenses for KSS to go to and from their lessons.
It's the main reason why Hashimoto Momoko decided to leave the KSS, and then audition as a challenger for Last Idol, since TWIN PLANET pays for the transportation costs even for challengers or temporary members that haven't become full members yet. It's also one of the main reasons why Goto Sayaka left, including the 8 hour roundtrip commute to the closest KSS lesson location at that time.
It's too bad that it looks like there isn't going to be a show that focuses on the people auditioning for J=J and Tsubaki. I really enjoyed the AmebaTV H!P 20th Anniversary Shin Member Audition show that they did a few years ago...