Flanker, while not exactly cheap, is rather on the affordable side.
Furthermore, unlike others, Russia willingly takes Indonesian barter.
I'd guess that's the problem with F-15EX - US takes hard cash only, and they don't really need nor have use for sudden excess of palm oil/natural rubber over that they already get from open market.
I'd think they would ideally want both, and dash from loiter for a good measure.
It's a basic set of requirements for an air defense fighter, otherwise it'd appear rather wasteful.
I.e. again, ideally a flanker, backup was the f-15in with CFTs.
They don't have plenty of airbases; scale of the map is quite huge; you'll need to thinly spread higher number of aircraft (which isn't and won't be high) across the entire country.
p.s. Kaan is also a heavy fighter. But it's so far away down the line I don't think it will affect current decisions at all.
Tbh, the reason why Indonesia dropped F-15EX because this. The bargain. And nowadays, Russia accepting the local currencies. Which is from economically side, this is actually "saving money" for Indonesia. That's first.
Second, before the MoU between The President (Minister of Defense, in that era) and Boeing happened, there was some serious talk between the representative from Boeing and American contractors with Indonesian contractors.
The Indonesian contractors wanted ToT, using scheme which called "Hilirisasi". So the Indonesia wanted some parts of F-15EX making locally using local's raw materials, before it was exported and assemble back into US.
Unfortunately, the representative from Boeing and US doesn't like that idea. The representative from Boeing wanted Indonesia exported the local's raw materials directly into US, make and assembled the F-15EX parts there, before imported back into Indonesia.
Indonesia just only buy the F-15EX, but doesn't have the ability to making parts locally.
For the Indonesia which had some serious trauma in the past about embargo, this is trouble. Losing the ability to making parts locally which means, you doesn't have the ability to maintain and flying the aircraft. Just rely on another aircraft parts to cannibalized.
That's the "real" reason why Indonesia dropped the F-15EX contract, beside of expensive.