I think extra attacks should be a passive exclusive to your highest level class and multiclassing should not give you the passive from a main class.
The fact is, for most classes the lv10+ perks do not outweigh the extra action that fighter and rogue give. It's way too much extra damage to have a second attack plus action surge plus all the proficiencies the class gives.
Monoclassing is super gimped compared to multiclassing. Really strong passives like extra attacks should be tied to the highest leveled class. You can then leave passives like the weapon/armor/combat maneuvers as the passive perks available to multiclassing.
Another problem with the balance in this game is that martial classes get way too many extra actions and enhancements from consumables and gear that casters, rangers, and druids simply have no comparable versions to. Oils, elixirs, potions, all highly favor martial classes. 95% of the gear is monk/martial class gear, and gear for druids and rangers is nearly nonexistent. In the case of ranger, I have not found a single ear that enhances my animal companion.