The point is to give someone vulnerable a full life, with safety and warmth.
From your perspective, time went by too fast; from their perspective, it was a long and peaceful lifetime, they were incredibly fortunate to have someone like you.

Also, there is an implicit assumption we carry around that to be immortal is some sort of blessing or state to aspire to, while it may very well be that being mortal is itself the blessing.

In my opinion, the highest tribute one can pay to a departed friend and companion is to again open one's home to another vulnerable creature and make him/her family.

Do it in your departed friend's memory and honor. If you could communicate again with them, you would let them know this is part of their proud and gentle legacy, to reduce suffering on the world - "Look what you did, by being who and what you were for me in life, you opened the door for someone else when their turn came."

This is what I have done, and do not regret a minute of it.