100% completion relies on Miiverse? That's pretty poor. Anyway, online multiplayer should stay up for quite some time (should - there's no guarantee) - the DS and Wii online was only terminated a little short of ten years due to some contract expiring.
I really do hope the online services stay. Would be completely unfair to end them after 5-6 years, DS and Wii had much more time and would of stayed longer if it wasn't for that company taking down the online services. But Nintendo seems to be so wrapped up in their little switch, who knows what could happen.