I mean the biggest reason is Nintendo would not be able to have the polish we are used to on their hardware. We also would have less new IP's. Something people complain about already. Splatoon and Arms would not exist as a 3rd party developer. If you have not noticed most major developers are already a 1-2 IP company these days. Capcom is no longer spitting out a new IP ever few minutes. They are mainly a 3 IP company now. SEGA same thing dropped from consoles and the company that once like Capcom spat out new IPs every few minutes is now down to only a few. Fire Emblem and Xenoblade series would be gone. Nintendo would do Mario and Zelda and a couple variants/spinoffs.
We need Nintendo making hardware for if only so they can continue to be making more than just a couple IPs. Before it became "fuck konami" Konami last gen was just MGS,CV and PES for the most part. That would be Nintendo's future as a only 3rd party company.