I never made a MS account, so I no longer own Minecraft since they stopped accepting Mojang accounts. Sometimes I wish I had just bit the bullet and made one so I can still own Minecraft—I know I can pirate it, but it's less convenient, and also I don't know how well pirated Minecraft works with multiplayer. In any case I've just not played Minecraft in a long time, and not since Minecraft stopped accepting Mojang logins.

I am surprised you can't transfer your licence to another account though. Since when they were making the switch to Microsoft accounts, they let you just transfer your licence from Mojang onto any old MS account.

Ultimately it's up to you. I guess in your shoes I would be more erred towards deleting just because of all that personal information sitting around. Of course you can't guarantee MS will "forget" it, but storage costs money, and they likely don't want to keep around all your old data when most of it is not very profitable data to have. In my case, in hindsight, I'm now erring on the side of wishing I had just made an account, since there'd be no other data tied to that account and I wouldn't have used it for anything other than Minecraft.