There's always a sad and happy feeling with love triangles. Unless the person left out meets someone new before we see the ending.

or we get the polyamory ending

I don't know, being with more than one person at a time seems kinda weird and unnatural for me, but if it's your thing I don't mind

Hmmm but I just realized I would like the harem ending for Watamote, I guess for these type of serious manga a polyamory ending would just seem like the easy way out to me

Ending with a polyamorous hook-up is a rather convenient solution to a love triangle, but I find that exploring an on-going polyamorous relationship would frequently make for much more complex and novel character dynamics than the typical "choose one" development.