There is still plenty of time to catch up with Bitcoin concerning hash rates. Even at the current hash rate, smart contracts are considered read-only and can't be modified. And while Ethereum is relatively secure at the moment, exploits may happen. But they could happen within any system. With important software still being developed, quite a few parties build/rely on Ethereum, and there is definitely continuous development of Ethereum itself. For the purposes of the Plutus app, the current system is completely adequate.