SWGOH.gg has a great meta data feature which shows the teams top 1, 10, or 100 in arena and aggragated its compositions.
What it shows is that Ben and Dooku aren't problems. They make up 11% and 22% of the top ten teams. So only 1/3rd. So based on the data there is pretty good diversity in terms of leads.
However when you look at overall teams RG, Rey and QG are on half of all top 10 teams. This suggests that EA/CG need to provide effective alternatives for the key skills that these toons provide.
Rey - GS has some presence but fotp and RP and maybe others should be buffed to be credible options for Rey
RG - stun is well represented out there. Auto taunt is not. Another one or two tanks should have this feature and a different secondary skill.
QG - the combo of good damage Oup and buff removal is very powerful. At a minimum give another toon good damage and buff removal
Now options for some of these exist in P2P toons as they should however if a goal is diversity then alternate FTP options should be developed.
Needing the 3 toons is not the answer as they fulfill neccessary roles in the game but giving viable options would improve diversity.