Yeah, and the cost to do that in this scenario is $72. Comparatively expensive, but you're flooding the network with money...

Anchoring a sidechain just increases the resources needed to tamper with it, as if it itself had a higher proof of work. It doesn't need to happen every ten blocks or any set number. As often as possible is good enough. Sidechains can even have sidechains. If the $72 was too expensive that chain can support hundreds of other chains anchoring into it, spreading the costs.

But most people would still just use exchanges and as long as the exchanges settle often enough that their government-mandated floats cover the difference, what do they care if the underlying network itself is slow?