Couldn't the biggest Bitcoin miner just switch all their resources to Litecoin for a few minutes and cause complete havoc?
That particular case is not possible because Bitcoin and Litecoin require different equipment to mine, so Bitcoin miners literally cannot mine Litecoin at any speed. But when two different blockchains use the same mining equipment it is possible for miners to switch chains at any time and thus a large miner could perform a 51% attack against a small chain. So small blockchains are not secure.
new projects to use blockchains for every little thing
Most of these projects would be better off running on top of an existing blockchain like Ethereum to avoid this problem.