Downloading roms is certainly not illegal, sharing them is.
From my experience most older Nintendo games are not available for purchase. I would gladly pay to play classic Pokémon for example games on the switch (or on Steam).
And nintendo will later rerelease those classic pokémon games in some form and rake in money off people like you (and me). Waiting is part of their market strategy. The NES and SNES classic editions sold out quickly iirc; when the time is right, a gameboy classic might do the same.