Is it necessary to replace old fillings with crowns eventually?

Not always.: It depends on the condition of each individual tooth. Usually if an old filling/fillings show no signs of caries and patient has no discomfort when chewing they don't need to be replaced.

Not at all: If the filling isn't broken - why replace it!

Usually not: Just because a filling is 'old' doesn't mean it needs to be replaced with a crown, or anything at all. If the filling is old and is failing for other reasons such as leakage of bacteria between the filling and the tooth, breakage of the filling, decay, etc, then it'll likely need to be replaced. Whether it's replaced with a crown, depends on the size of the filling and the extent of the damage.

Crown vs filling: It depends how big the filling is. If therer is unsuported tooth structure, that can leed to a brocken tooth and tooth loss, than get a crown to prevent fractures. Otherwise most dentists except some holistic dentists say leave it alone if there is no decay or issues.

Onlay vs crown: Another option for a big failing filling can be an onlay instead of a crown. It allows for a more conservative procedure. Think of an onlay like a puzzle piece that just replaces the broken down portion of the tooth, leaving the healthy part intact.

Not always: Many fillings can last for several years. A silver filling can last 30+ years and still be fine. Problems can arise when recurrent decay forms, or when cracks develop adjacent the filling. If there isn't enough healthy tooth structure left, a crown may be the best option.