Both heroes were reworked because their core designs didn’t fit the game. Just, in general.
Mercy’s ultimate was flat out bad for the game. One ult could up and delete the efforts of 6 others, and was either impossible to stop or utterly useless with very, very little middle ground die to a combination of Mercy’s squishiness and her mobility.
Pre-invulnerability frames, it was useless.
Post-invulnerability frames, it was unstoppable.
It was more powerful than any enemy ultimate combo provided the combo didn’t kill the Mercy.
Grav-dragon < Mass Rez
Nano-blade < Mass Rez
Blizzard-Noon < Mass Rez
Shatter-Blossom < Mass Rez
[insert ult here] x 6 < Mass Rez
Without an effective Mass Rez, mercy was worthless. With one, she was a must pick. There was no in-between.
Sym was the exact opposite. Because of how her kit was designed she was utterly, utterly worthless on attack and only marginally useful on defense. Her auto-lock beam was either oppressive or worthless depending on enemy skill, though unlike Mass Rez it did have middle ground.
Symmetra 1.0 and 2.0 have the distinctions of being the only heroes with an effective 0% pick rate in pro-play, and not just S1 OWL. OWL, Contenders, and the 2016 & 2017 world cups saw zero symmetra play. Additionally, her pick rate above silver was nearly zero for two full years.
She needed her reworks very, very badly.
Do their original fans have a right to complain?
You obviously have every right to complain as long as you don’t violate the forum TOS. However, I don’t think your complaints have much value.
The states of these heroes were just bad. In Mercy’s case, while mass rez might have been more “fun” than Valk, it was absolutely toxic to general gameplay. From hide & seek to tempo rezzing, it completely shaped the ult game by itself. It was too powerful to be left alone.
Sym, may have been more fun before for some people, but if a hero sees zero usage it means it is a waste of the manpower used to build it. I’m sorry that you “lost your hero” but the rest of us deserve to have a hero that we can enjoy, and there just wasn’t enough of you to outweigh the rest of us.
If you want a simile example, we could equate the availability of old Symmetra to the availability of a bucket that only 0.2% of the population could use to drink water, or something. To which the old-sym fans would respond, “but this is just a game!”, and to which I would respond, “that we all help support.”
Symmetra wasn’t just “a little weak”. She was a terrible hero whose design was never going to work. She was designed along principals that the devs thought were going to drive the game, but ended up being based on completely incorrect assumptions.