I completely agree with your suggestion. A lot of the game's systems and ideas are very outdated.
The achievement system is one of them, especially when the character gets deleted, the game just lets you lose all the achievements in a matter of seconds, after years of work.
The systems are designed in a wrong way and it's probably a big reason why people hesitate on creating new characters, that's probably the reason I have never touched any other class than Barbarian in 3 years and my only alternative is quitting, instead of grinding all over again on a 2nd character.
Not only that but many people do care about cosmetics, there are a lot of nice cosmetics from Battle Pass ( Haircuts / Costumes ), and if you want them on multiple characters you have to spend $ and skip the Battle Pass all the way down, there is no way you can grind the tasks on more characters on the same account before the 40 days are over.
After making a new character, will I get these? No. Same as the achievements, the systems are poorly designed, and players are forced to start from 0 and even miss the Grind from 1st character because the content doesn't exist anymore.
Therefore I agree with @Arthas , Achievements and anything Grind-based should be rethought and implemented differently like in any other MMO RPG, otherwise the Player base will only go down, and never up.