11. Do you agree that in situation when we have an array of virtual currencies, each one tailored for its own niche market, with their total value fixed (I.e. that total value of all altcoins is never above 21M BTC and you'll never lose on exchanging from altcoin to bitcoin and back), chances of them getting mainstream adoption would be considerably higher than now?
12. Do you agree that sidechains offers such a solution?
which was my point number 4.. sidechains will cause even less people to want to run full-nodes
No, you don't get it. Neither miners or users have to worry about sidechains that they don't use, because you don't need to download a chain you don't use. Points 2 and 4 are invalid.
But the franky1 point that you split the hashing power remains. This splitting makes every sidechain less secure than if you have one scaled blockchain.
And who decides how many and what properties of the sidechains? You let free market decide like everyone can create one and he will compete to get haspower to get security for his sidechain (aka altcoins, and again, making average sidechain hashpower only lower) or just Bitcoin developers will have the final word?