If you have just realized that now, then it’s a belated Welcome to the club of enlightenment. As they say, better late than never.
TBH, one could already ascertain that there is an imbalance among heroes. Season 1 heroes alone, if you have started playing prior to Season 2, among the classic legendaries, the least loved are the slow purple and nature ones, while the fast snipers are very much coveted, with several players maxing 2 or 3 of them, some maybe more. The imbalance got accentuated with every new release of heroes, may it be from events, seasons, seasonals, and even HOTMs.
This game is designed for players to play and grind for years. Ask yourself, why do you buy clothes and shoes every now and then? Most common answer is because you want new ones to replace your old ones by reason that they are pretty worn and very much used. As with everything else, the young replaces the old.
Just my opinion, I think that the natural course of action is for SG create new heroes that are a bit powerful than the old ones. I think they have done a wonderful job transitioning from S1 heroes to S2 heroes as they increase in power and ability is not that very much that apparent. However, from S2 heroes to S3, the differencr of power, stats, and skills are so much noticeable. Much more so with ninjas and the additional heroes from the monthly and seasonal events. And now, S4 heroes.
In the end, if you are bored and find no enjoyment in the game anymore, there is no other better recourse but to move on and find greener pasture somewhere else.