I agree with what you say right at the very beginning of your post. That S1 heroes are being outclassed by the new series heroes and event heroes being introduced.
The fact that they (S1) form the majority of pulls from the portals makes matters worse.
By the time that the new stuff arrives as detailed in the 2022 sneak peak, I am not sure if many S1 heroes will be relevant anymore.
But not sure if or how it can be balanced within the game
I think all the S1 heroes should gain equally rather than based on how long I’ve had them. And there are some S1 heroes that need this far more than others.
Costumes were one way to keep S1 heroes relevant (at a cost, naturally, this is SGG after all). But that has now been negated with the costumes coming out for everyone, putting S1 heroes back to square one.
I think the question is what do you mean by relevant?
My bench is primarily S1 and I sit quite comfortably in the 2500-2700 cup range. I am usually in the top 3 in my alliance in wars, often top. But I have no ambitions to be at the apex of the game. I play this game for enjoyment and stress relief, not for domination and the accompanying stress creation. Are my S1 heroes relevant to me? Yes. Are they relevant to being at the top of the game? No. And they haven’t been (except for certain costumes) for a long time now. It is not to SG’s benefit to keep S1 heroes relevant to the top of the game. That cuts into their bottom line. Until that changes I do not see it happening, no matter how many times (and there have been lots) we here request it.
Doesn’t mean I wouldn’t welcome it. I was mainly pointing out here that basing the improvement on how long I have had a hero on my bench is not IMHO fair overall. And I have had Lianna for about 3-1/2 years now.
So, I can speak clearly to the S1 costumes losing ground as the non-S1 costumes move in. Plus, half of my costumed 5* heroes rarely see the light of day. I prefer plain old Domitia to either c.Quintus or c.Obakan, and plain old Marjana gets first choice over C.Azlar.