This assumption is made in virtually every publication put out by the Watchtower.......(that Adam and Eve would have lived forever if they hadn't sinned, and that we would be living forever also)
Something I've never seen discussed at all, though is ......what if they remained faithful....... do you really think all their offspring would have followed their example?
Even some of God's angels in heaven disobeyed Him........I doubt that every single human from that moment on would have proved righteous and remained perfect.
We can only speculate.......but it seems that eventually you would end up with some Cains or Nimrods, who would even have killed any perfect people they could.
I think it was inevitable that we would end up with all the imperfection we have today........It's a big test.