I would be interested in why
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I generally trust my own human error factor enough to keep a backup of most of my old wallets. And in some cases I re-used addresses, so these wallets should *not* be deleted because you're more likely to make a mistake if you delete the wallet or seed phrase. I have to add that this also is true if you shared an address somewhere, for example to a family member, a recurring customer or business partner, etc.
I thought you're going to ask for it.
Also, valid and important points you added to your post.
First, if you never re-used addresses, you haven't shared addresses of this wallet with other people, and have thouroughly checked that your wallet was 100% empty for a year or so then there is imo no reason to _not_ delete it.
I find it harder to track or keep track if I had ever re-used or shared addresses than keeping a lowered security backup. It eases my mind more, knowing, I still have a backup, than thinking about it if I can completely discard a wallet or if I will ever need it again. That's maybe me and YMMV.
1) if you open it accidentally, you could leak information about you. For example, if you have an Electrum wallet from 2012/13, and are checking all your wallets on an old device, connecting to the Internet. Electrum servers are often run by chain analysis companies, or by other people doing chain analysis even for worse reasons (to sell them to criminals for good targets for $5 wrench attacks for example). If you are not using Tor and the device is on the same home network (router) than the device with the main wallet, then your IP address tells the chain analysis guys that you owned Bitcoins in 2012/13, making you a possible target.
Doesn't apply to me as I deliberately run my own full archival Bitcoin node and Electrum server to avoid having to expose my SPV wallet's addresses to foreign Electrum servers. That's a "no count" for me in particular.
2) if a thief finds your seedphrases and sees you had an old wallet, then he could assume that you are richer than it seems - and plan a even bigger heist.
Point 2) is the only one that I can agree to the most, especially when I argued that you can lower the storage security for abandoned wallets.
From past bad experience, I tend to document quite detailed what any of my wallets have been used for and if they are still live or not. Of course, I don't keep this documentation too openly where I live. I also don't leave any of my hardware wallets too easy to find. Chances that a thief finds them, I would consider low, based upon that a thief commonly doesn't spend a lot of time in a victim's place (unless the thief knows, nobody is coming home for long and he won't be interrupted or cought).
I use mnemonic passphrases as mitigation against exposure of my mnemonic recovery words and, of course, those aren't stored alongside my recovery words.
My redundant remote backup storage isn't also easily exposed, but I have somewhat less control over it than at the place where I live.
Having lower security for the backup of abandoned wallets could mean e.g. to refuse to have a redundant remote backup of those. I don't expect to ever have to use them again, so if my primary backup burns down, I could afford to permanently loose the sole backup of my abandoned wallets. I wouldn't like it, but it won't hurt me.
Having an intruder in your own place is already a bad breach of your privacy and I hope I'm spared to have ever to experience this. Fingers crossed! Where applicable, I use full-disk encryption on devices which I use for value related stuff, crypto coins, e-banking and such. I can't defend against anything, but I try my best to mitigate the most likely risks while still trying to keep things not too paranoid and still enjoyable.
3) if you want to delete evidence about old transactions for privacy reasons, for example to prevent curious family members to check old crypto transactions.
I consider this a "no point for me" case.
I keep a backup of the wallets I no longer use in a specific folder on my computer for that purpose.
You could make it a bit safer e.g. by using an encrypted archive. I try hard to separate my daily internet shit device from a device where I do crypto coin stuff. A bit less convenient but feels safer to me. I avoid (a)social media crap and clicking on links from strangers as much as possible and I know a thing or two about computer security which helps to avoid doing stupid things.
