Im not woried at all.
We seem to add a lot of players every year, some of them in the prime (Khedira, Pjanic, Sandro, Higuain, Matuidi, Tevez, Cuadrado, Mandzukic...), some young and promising (Pjaca, Morata, Lemina, Sturaro, Rugani, Mandragora...), some experiensed and over 30 years older but WC players (Pirlo, Alves, Evra...).
Why are people woried its beyond me. Even our oldest players have at least 1-2 years on top level (Buffon, Barza, Alves), maybe not as starters, but for sure a bench options. The rest of them (Giorgio, Claudio, Mandzukic, Khedira, Matuidi.. ) as I said, have at least 3 more years at top level. Add here the players we will get in the future, its not like we are gonna stay with the same team every year.