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You're logic is good but the implantation of it just isn't viable, from each corporation having their own chain and it still being open source, to the bitstream team giving seal of approvals for tons of chains without making changes, and for chains to auto convert and with all these things a small fee, the man power to get all this to happen and avoid bugs, scams and high fee's is just insane when compared to increasing the block size and using alt coins if you want other coin styles.
Just FWIW, I have a decent hands-on insight into how corporations tend to get software. They very rarely write core systems in-house, and usually contract out even fairly basic adaptations. For tricky cryptographic code it will almost always be the case that it is commissioned from experts.
If Blockstream develops various sidechains for various customers there is some chance that they will demand to have it be open-source. There is also a fair chance that in many cases the client themselves will wish to have it open-source as a method of instilling confidence.
If a sidechain is closed-source I still might use it for non-trivial needs if it were released by some entity I trust, but this would be a strike against it in my book (though I'm a bit of an odd-ball in these things so it may not matter much to most people.)
I would even use closed source from an entity I didn't trust when only trivial values which I can afford to walk away from (and laugh) are involved. Indeed, I entrust significant sums to Coinbase at times (out of necessity) and I have no idea what their back-office looks like. From my experience in the commercial software industry I figure it's probably a mess held together with shoe-strings and bubblegum. Most closed-source commercial software fits that description.
