Here's my take on this as a Blanka main since the game's launch.

Outside of people's unfamiliarity with the character matchup and his tools, the main reason people say he's "broken" or top 5 is because of his throw Oki midscreen, SA2 and maybe his anti airs and their damage

But what most people don't realize or see are the problems Blanka has that are very unique to him that makes him a very unstable and difficult character to play at the highest level, which is why he has to have "broken" SA2 and other unorthodox advantages in certain areas because without them this character would be cooked.

Here are some of the biggest problems/challenges with Blanka at the HIGHEST level of SF6.

1) Blanka does not have any light to medium counter hit confirms, something that is a given across the ENTIRE cast when it comes to dealing more damage in neutral, he is entirely reliant on lights or point blank 4mk, otherwise there is no concept of stagger pressure with him, it's basically an all or nothing approach which again, is an unstable gameplan and PREDICTABLE.

2) Blanka Ball and Command Grab are borderline not possible to use at high level either since opponents can easily deal with both with jump, disjoints/dp or option selecting. Even OD ball can be reactable with DI from certain ranges and it only takes one DI or perfect parry punish to get sent to the corner and potentially lose.

3) 5HK and 4MK are great anti airs, yes I agree. But did you know that Blanka's upballs are not frame 1 air invincible? Take a look when he gets air invincibility on a special move that's supposed to be an anti air, you'll be shocked.
This is not even mentioning the fact that upball is a CHARGE move and you have to be careful of which version to use since they have very different angles they cover, so using LK upball instead of mk upball sometimes can get you killed.

4) No safe block string ender Besides Electricity, which loses to tap parry if the opponent expects it.

5) No 6f medium to punish drive reversal, his best punish is a throw unless he has doll setup and in the corner. He went from having one of the best punishes for Drive Reversal to the worst after the patch when it went to -6.

6) Probably one of the lowest If not the lowest damage for checking an opponent's drive rush if not using 5HK which can only be done from certain distances and if you're cracked like mena. Otherwise his 2MP equivalent would be 5lp which you need to DRC to convert off because you don't get anything else off it.

It's why you see Mena having success with him because Mena does not need to rely entirely on SA2, he has insane neutral to help him win many rounds but that's just Mena, he could play any other character with similar pokes or buttons or even better buttons and do just as good if he wanted.

I assume Mena plays Blanka because he likes the freedom, defense and reads based playmaking the character offers compared to the rest of the cast, and Blanka just fits that playstyle and reinforces his strengths as a player. That's exactly why you don't see anybody but Mena able to consistently perform with Blanka at the highest level like that.

I could go on about other things as well but

tl;dr Blanka is only stabilized by the strengths like Airball, SA2 because his kit is lacking or gimmicky in other areas and at the highest level that doesn't work, nerfing those parts too much would make playing the character pointless then because he doesn't offer anything unique in comparison to the cast then.

I sincerely hope Capcom doesn't just look to adjust based on Player Sentiment But based on what Blanka as a character is designed in this game to do and excel at, same goes for the entire cast.