Considering the apparent serious implementation and backing behind the sidechain, the market starts using it and finds considerable value in it. price of BTC goes UP
why would price do this?

People find value in the ability to make anonymous transactions. Buy BTC to use particular sidechain. Price of BTC goes up
ah, but in your world it's not possible for a sidescam to cause the price of BTC to go down?
No.
Is the price of BTC going down because of altscams? In fact, a successful sidescam might push BTC's value up since it would essentially increase the scarcity of the remaining coins.
wow, listen up everyone! SC's, whether scam or not, can only make the price of BTC go up! never down!
this is so blatantly economically naive that i am forced to give brg444 a pass; after all, he's a 24 yo kid with a self admitted shitty job.
otoh, please explain to all of us in detail how SC's, real or scam, can only make the BTC price go up?
The way I read brg444's comments are; Since SC's only add functionality to the BTC main chain, that functionality can only be beneficial to the main chain and increase the main chain's value. If a SC was created who's functionality was not useful and does not add value, then that SC does nothing for the main chain but also does not have a negative effect. It just simply has no impact.
In that sense SC's can only add value to the main chain. If their features are useful then they add value, if they are not then they have no impact.
You can agree or disagree with that, but it's not economically naive.
It is economically naive to fail to consider that a feature that adds value in one economic condition, may subtract it in another, and that movement of money may follow this with a lag, sometimes a very significant lag due to practical constraints.
Further naivete would be in imaging that this is not happening all the time, and that this effect is commonly exploited. "Economists" here are contemplating only static ideal cases, and implementing regulatory changes in spite of these concerns. Changing the code is changing what ultimately regulates Bitcoin.
With this particular change, it is not so easy to undo it if it turns out to be a mistake. Many Bitcoin may be destroyed. People will make mistakes of trust.
There are many new risks with SC. Lets address them rather than pretend that they don't exist.