The Cavs lost because of their big 4
Game 1 +-
James -22
Irving -17
Love -11
Thompson -13
(since you mentioned Korver, Jefferson, and Deron - I included them)
Korver -8
Jefferson -7
Deron -8
Game 2 +-
James -11
Irving -17
Love -8
Thompson -18
Korver -3
Jefferson -7
Deron -8
Game 3 +-
James +7
Irving -9
Love -11
Thompson -6
Korver +3
Jefferson -5
Deron +4
Game 4 +-
James +32
Irving +7
Love +18
Thompson +3
Korver +3
Jefferson +13
Deron +3
Game 5 +-
James -13
Irving +4
Love -23
Thompson -7
Korver +0
Jefferson +3
Deron +2
So here are the series totals, with total minutes
James -7, 212, lost 1 point every 30.29 minutes
Irving -32, 202, lost 1 point every 6.31 minutes
Love -35, 161, lost 1 point every 4.6 minutes
Thompson -41, 132, lost 1 point every 3.22 minutes
Korver -5, 97, lost 1 point every 19.4 minutes
Jefferson -3, 83, lost 1 point every 27.67 minutes
Deron -7, 61, lost 1 point every 8.71 minutes
For the record Golden State scored 608 points, the Cavs scored 574, so a difference of 34 which over the 240 minutes equates to GS gaining 1 point every 7.06 minutes. So those 3 bench players you called out actually did better than the Cavs team as a whole and Irving, Love, and Thompson were well below the team average.
The Cavs lost because outside of James the Big 4 was terrible. The Big 4 aren't old. They aren't run down, they are just greatly flawed players (outside of James). If you want to argue that James will leave because Irving, Love, and Thompson aren't good enough, then that is a fine argument, but if you want to claim the Cavs aren't winning because there bench is old and isn't any good, the numbers just don't support that position. The Cavs quite simply lost because Irving, Love, and Thompson just weren't good enough and they may never be good enough because Irving and Love are poor defenders and Thompson is a terrible offensive player.
All that said, if the Cavs don't choke game 3 away, that is probably a 7 game series (I'd expect GS to win) and the outlook is probably a bit different in Cleveland if they pushed them to 7. But the Cavs did choke game 3 away and ended up losing in 5. The Cavs need a top level player that is at least good on both sides of the ball so they don't have to hide someone else on one end of the floor. That is why the Cavs tried to move Love for George. George would have been the perfect for the Cavs because he is a high level player on both ends of the floor.