Moving photos from one service to another is doable, but it requires a LOT of effort from the average user (relatively speaking). Even if another company made an "import instagram photos to service X", the perceived effort of moving would prevent most users from finding such an import feature and making the move.

Photos are a pretty solid natural barrier that could protect Instagram while they made that long trek.

That being said, stickiness and perceived lock-in with photos doesn't make the trek any less arduous.