I like the idea of making animal companions follow the druid progression.
For a class that’s specifically combat oriented, often melee, the current progression of half ranger level makes the companion very fragile compared to the ranger. They are practically disposable.
Under current rules, a 10th level ranger has a companion as a 5th level druid: +2HD, +2 AC,+1 Str/Dex
A 20th level ranger has a companion as a 10th level druid: +6HD, +6 AC,+3 Str/Dex
Sticking with the basic companion options, you could have a 4 HD wolf at 10th (26 hp) and a 8 HD wolf at 20th (52 hp). Not likely to be around for long at those levels.
Under the proposed change, the wolf at 10th level now has 8 HD, and 14 HD at 20th. Much more sustainable.