I do think M&S should offer Experian/similar and should have done more to protect our data.
That said... you can get my name and address from the electoral roll (if you really wanted to). My email address and phone number is vulnerable to spam now but I only ever look at emails I expect, and I just block nuisance calls. Date of birth has me concerned - and I wish I'd lied when I signed up - but to open a bank account they'd need my exact address history and my full name which I never use online anyway.
I'm not saying ID theft doesn't happen - it absolutely does and causes huge distress. But to open a bank account in your name, they'd usually need address history, which these hackers probably don't have. To open it under a new address, they'd probably also need proof (like gas bill) with your name on it, and a form of photo ID. They could try to pretend to be you to get access to your bank account, but they'd likely be missing a pass phrase/your mother's maiden name/similar.
I have been anxious about my personal data for years but when you look at it and see that your data is absolutely everywhere, it's quite hard not to resign yourself to it. There's a possibility, if you're like me and have lost things like driving licence in the past, that some nefarious person pieces up all the fragments and pretends to be me - but I'd find out eventually. It's not ideal, and I could do without the stress, but I'm just going to chalk it up to experience.
There are 8+ billion of us in the world, statistically it's very likely our data would be shared. Frustrating, but every time you build a system, there's a computer working to break it. M&S probably couldn't prevent it 100%.