Well Hamilton is an all time great. Ferrari are completely justified in employing him. As Alonso is proving, age is no factor as long as you maintain fitness.
I don't buy that, Hamilton was great, not anymore. Alonso is very competent, sure, but both definitely not suitable for a long-term project.
I would understand if Hamilton would be a seat filler until a star rookie woud be taking over, but kicking Sainz who is delivering very good performances is a bad idea.
Hamilton is not supposed to be a long term option. He is possible fairytale story if he wins the championship in the 3-4 years he is at Ferrari.
Because sainz is as good as he is, Ferrari don't need him when they build a championship winning car because him and leclerc would have made the same sort of pairing as rosberg and Hamilton leading to intense rivalry within the team.