I think the system could be improved with two steps that would positively impact the bulk of the player base without throwing out all of the work that elite players have put in.
The first step is to have a staggered AL boost, such that earlier levels give higher stats and it drops as the AL levels increase. I would propose AL1-A10 give a 3% boost, AL11-AL20 give a 2% boost, and everything beyond remains the current 1%. This early incentive to AL's would make the system more appealing to the majority of players without harming higher AL players (it'd actually boost them even higher).
The second step is really the most important part of this- have AL's between different classes determine an overall AL bonus. What I mean is have a player receive a portion of the AL bonus for a given class dependant on how many other classes they have at that same AL with let's say the max bonus occurring when 4 of the base classes (not celestials) in a given tier are at that AL. This would allow players to continuously grind out their main class but promote diversification to make the AL's mean more. The largest bonus would come from the first 10 levels of a given class, followed by the next 20 and leveling out after that point. High AL players would still receive a fairly large bonus from their work, but it would be brought down by this move to make it not an overwhelming advantage against the bulk of players.
Here's an example for step 2:
(all numbers are purely for demonstration, not set proposed values)
A player has the following classes with AL's:
Gilga- AL10
Hera- AL12
Realm- AL22
Deity- AL41
All classes would receive 100% of AL bonus through AL10 (30%)
3 classes receive 75% of the bonus for ALs 11 and 12 (+3%, 33% overall)
2 classes receive 50% of the bonus for ALs 13-22 (+9%, 42% overall)
The highest class receives 33% of the bonus for ALs 23-41 (+6%, +48% overall)
Personally, as someone who's turned casual from my more active days, this is a system I would enjoy.