If you think any SU series aircraft (post SU-27) is an "unproven" design, you need to wake up. The SU series is one of the most lethal out there. The US trains with Indian pilots against the SU's as they are a credible threat.
If Indonesia gets this, they can setup their aviation industry at a much advance level than where they are now. Having the state of the art SU-35 as a baseline only means they are starting from today's tech and they can advance features on their own as they like. This is HUGE for the Indonesian defense industry. Saying no to this would be idiotic to say the least!