I don’t think that a lasagna’s structure lends itself very well to being held; if you’re willing to rule out soggy sandwiches because you aren’t able to hold them in your hand, I think lasagna would be excluded under the same logic.
I don’t buy it. The fundamental nature of the taco doesn’t alter because of one crack. It has the same ingredients and the same structural elements arranged in the same fashion; if you can have things accidentally becoming sandwiches when they weren’t before, simply because there’s a structural failure that changes one connected piece into two discrete pieces.
For another example: if you tear a tortilla into two pieces, and wrap one half around taco fillings into a U-shape, you have a soft taco. If you wrap the other half around the top, it’s now a sandwich… despite, for all intents and purposes, being now completely indistinguishable from a wrap/burrito?
…And I still think that it’s ridiculous that “a filling between two discrete bread-like objects” as a definition for a sandwich includes Oreos and wafer-cookies but not any kind of sandwich where the two halves of the bread remain connected (hot dogs, subs, etc.).