There is no noticable decline in Djokovic. Also, if Federer could win slams at 36, so can Djokovic with much better movement, stamina, mental. All he needs to do is stay hungry
ur assuming no one in the next 5 years will become "dominant"... fed caught a break with djokovic the "dominant player" going AWOL for 2 years.
Djokovic is going to have several challengesr and is already struggling in big finals atp 1000, world tour finals against these up and coming players. Happening in slams is just a matter of time.
Its how sport works.