I concur the 2.5L Subaru engine (non-turbo) has head gasket issues from 1996-2004(not present). However they seem to have stopped in 2005 where the engine design changed a bit. This is even evident in Consumer Reports which shows major engine problems for all the models (except turbo) only up to 2004. The major engine problem is the head gasket. The rest of the car earns high marks.
The $1500 while quite steep could be worse like a sludged motor($3000-$5000) of early 2000’s Toyota or early 2000’s Honda automatic($3000+).
Subaru should have fessed up more and made the warranty 8yrs/150k not the 8/100k.
The coolant conditioner a diluted stop leak that gives Subaru insurance to the problem across all models.
The turbo 2.0L(2002-2005 WRX) and 2.5L turbo(2004-present) are immune to head gasket problems in any significant #.
That being said my family has 4 Subaru’s with the effected engine and anywhere from 100k-200k and not a single cooling problem. They do maintain the cooling systems though. So your logic they all fail is not true. I just believe it is a high percentage like 30%+ not 100%.