Game Boy didn't actually have steady sales ... sales were collapsing around 1995 for the Game Boy .... this why the Virtual Boy was released. By the fiscal year end of the 1994 year (March 1995), Game Boy sales were down to 5.5 million WW or so, down from its peak of approximately 16.5 million about 3 years prior.
When people say "well it was just fucking random Nintendo released the Virtual Boy" ... no it wasn't. Yamauchi was unhappy with floundering Game Boy sales, this is why Virtual Boy was fast tracked into production to make up for diminishing Game Boy revenue. Nintendo was working on a Game Boy successor codenamed Atlantis, but it was bulky and heavy and would not be ready any time soon.
What ended up happening though is of course Virtual Boy flopped and then Pokemon came out of nowhere in Japan in late 1996, gaining momentum in 1997 and that gave the Game Boy a completely unexpected second wind, but that's something that happens once in a lifetime.
It was only due to the Pokemon crazy of the late 90s that Game Boy was given a second lease on life and Nintendo released Game Boy Color instead of Atlantis.
Here is a Project Atlantis prototype by the way ... look at how huge it is compared to a DS Lite.
