Tell me you've never worked on a game without telling me you've never worked on a game.
Sometimes (and by that I mean 99% of the times) you just don't have the funds or time to make assets for a prototype, you need a fast iteration cycle to nail the core mechanics. You need to learn as soon as possible if the game works or not, and while doable with Mario and Pikachu sprites it's much better if you can get a feeling for the atmosphere as well to see if it matches. This is the first hours of game development, after the initial GDD and before you even consider the full scope of the game.
But I don't expect someone who sees in black and white to be able to understand the many scales of grey there are in this. Using assets without copyright for them is also wrong, yet that's a daily occurrence in prototyping, because you're not going to spend time and money building assets for an idea to check if it's worth investing time and money into, that defeats the purpose.