Yes but then you create two competing chains that work against each others network effect.
It is also different from sidechains because then you have to decide whether or not to use the feature of, say, your Monero spin-off.
The competing network effects may be a drawback, but not that much compared to the peace of mind of knowing your wealth is secure no matter which platform wins.
As you have mentioned above, you can't benefit from your wealth unless you spend it. The point of sidechains is not only to ensure your stake in the ledger but to be able to use it in different ways that are not possible on the mainchain.
Right, but the simple act of converting your stake into a house, bars of gold, cars, etc. doesn't require any special functionality beyond what Bitcoin already is.
The store of value function is surprisingly way more valuable than anything else, like by an order of magnitude (consider total world gold value). It's not just another way to use money.