Given how time travel works in this world - new timeline erases the previous one completely - that means New Dave never meets Klaus in Vietnam.
When they go back to 2019 the Dave that Klaus fell in love with doesnāt exist anymore.
Thereās no ghost smiling at him.
Thereās no eternal afterlife together.
For New Dave Klaus is literally some weirdass cult guy he met briefly a couple of times and punched in the face once.
Death no longer is something Klaus can accept with calm and dignity because thereāll be no Dave waiting for him on the other side. Once again, heās alone.