You're the third person to suggest this post-canning mustard treatment. But you are polite and cordial about it instead of outwardly dismissive, and this is appreciated.
I'm no stranger to mustards. I've got a few varieties on-hand at any time. My favorite American yellow mustard is in rotation at all times, along with at least one bottle of backup mustard in the pantry. There's also always a tickly sort of Dijon in the fridge and I keep a random-brand discount store jar of coarse-ground imported mustard (usually from Poland) available.
I'm out of the extra hot version of Löwensenf, from Germany, at the moment. I need to fix this, but it's hard to find at a reasonable price. I'll find more of it soon enough.
When my favorite mustard once became impossible to buy in sane quantities, I found myself ordering an insane quantity: A box of six one-gallon jugs was easy to order from a local food service supplier and it was surprisingly quick and inexpensive to get it done. (Yes, six entire gallons -- nearly 30 liters -- of mustard, all at once. I bought extra pumps and made a couple of kits to give away to some people I know; the rest was for me.)
Anyway, sardines: I don't know if my grandparents ate sardines or not, but I doubt it. Out of countless meals and snacks, I don't really recall ever eating fish with them at all.
I cut my teeth on sardines at the kitchen table with my dad: He always kept a few tins of Brunswick-branded sardines (always in mustard sauce) in the cabinet. One day, when I was very small, he talked me into trying some, which started with a lesson in winding open a can with a key. After getting them open and overcoming a moment or two of confusion about the strong scents and flavors along with the texture (edible bones? wtf?), I found that I liked them quite a lot. We then frequently shared a tin of sardines together, just picking out little bits of fish with our own forks, which was lovely until things changed as I became a ravenous and disdainful teenager.
And of course I've tried putting mustard on more-plain varieties of sardines. I've tried it over and over again.
Mostly, I've previously experimented with this technique with sardines from Aldi: There, 3 or 4 varieties share the same SKU and arrive packed together in equal quantities, all in one box. This leads to the most-popular kinds disappearing from shelves while the less-popular kinds can become a sort of permanently-annoying surplus. My observations while digging through stock at Aldi suggests that mustard sardines are the most popular in my neck of the woods, because those are the ones that are always hardest to buy here.
So, thus: They historically have always had a ton of sardines in water and/or oil. And they were always priced right, because Aldi. And I always have (what I consider to be) good mustard available. It was obvious to try putting the two things together.
It makes me happy to hear that you think it's better this way, and I do wish that my own tastes were so easily placated.
For my part, I think it's far better when the mustard sauce and the sardines have both spent a lot of quality time together while sealed up together inside of the same can. I find that the flavors meld together beautifully, with the flesh of the fish becoming deeply mustard-flavored while the mustard goo becoming strongly sardine-flavored and a bit gelatinous. I enjoy this quite a lot as I scrape the last gloppy bits of canned mustard and fish out of the tin with a fork.
I find that this experience can't be sufficiently replicated by buying the two things separately and then combining them at serving. The packaged whole is greater than the post-purchase sum of its parts.
But, again: I'm pleased that it works for you and that it sounds like it might even be preferable.
It's always good to find ways to maximize one's enjoyment of sardines, and I think I'm doing pretty well on maximizing my own enjoyment.
The distribution SNAFU will either sort itself out, or it won't. :)