You can't really compare Stirke to Impulse on he ability to change parts. Impulse is pretty much built completly from smaller parts while Strike was an MS that had different battle armaments. Anytime a piece of Impulse is destroyed it can be replaced midbattle, with Stirke the only parts replaceable are the batterypack/specific weapon set up. For example, if Strike were to lose a leg in battle it could not be replaced on the spot like it could with Impulse. This aside I don't believe this is the only reason why Shinn has never lost a MS. I think that reason is that so far he has only fought 2 people who could possibly beat him. The first was Kira who was not trying to kill Shinn, and the second was Athrun but he was only in a grunt suit at the time of the fight. If anyone one is going to defeat Shinn I assume it will be one of these 2 since a side character beating a main in battle would be a little weird at this point.