Well the Bitcoin node doesn't have to execute the scripts. The people that care about the scripts would do that and they could easily verify the validity of the scripts. The effect would be something like a side chain, but embedded in the chain itself.
I don't know if it, or something similar, is a good idea or not, but if it valuable and economically viable, it will happen.
The point of using a side-chain is to avoid spamming the mainnet with anything that is not a "transfer of money". It also has other benefits such as being able to have a clear and solid consensus rules that is enforced by a clear and solid network instead of some abstract "agreements" (instead of rules) defining an abstract protocol.