Sidechains is a creation for the greater good of the ecosystem. Blockstream do not own sidechains.
They are simply a group of competent enough developers that have the skills to build on top of it.
Blockstream is way more than this. they include 40% of core devs in addition to 3 of the top committers. they are in a position to influence all future decisions regarding upgrades to Bitcoin, and that includes blocking competition that could make SC's obsolete.
their incentive being to sell as many SC's to any willing buyer. that is what a for-profit does.