Decentralization brings an obvious benefit - if power is consumed near generation, you need less infrastructure to distribute it, thereby lowering costs. Once we start adding large numbers of EVs charging at home, our energy consumption will tilt more and more towards the home, making generation and consumption on the spot much more efficient and useful.
Another benefit of decentralization is grid resilience. Removing single points of failure by distributing them means that large scale power outages (while already infrequent) would become less frequent.
There's the physical security aspect of this, which is as you say, hard to take down when it's decentralized. However, as power generation gets more distributed, we'll naturally start seeing more (if not most) of these pieces of equipment be controlled over networks (private or public), and securing distributed infrastructure from a software standpoint is still a hard problem.
Just look at the state of IoT security today. It's quite bad, and that's not even realistically including nation state attackers in the threat model. I don't expect this to go well with a decentralized grid, at least not for a while initially.
IoT is a raging tirefire. It's hard to even imagine how bad the security situation is.