Yeah, I see what you're getting at here, Metal has it worse than other characters in the franchise because instead of just being relegated to a background character or have all his development removed. he's just kind of there and every single time he makes an appearance it's always the same thing, it's always just a race.
Now that makes sense and is in line with what he originally was meant to be but at this point, it just doesn't feel special anymore.
And that segways into the next thing I want to talk about, now I noticed that you brought up Sonic Heroes and what they did to Metal Sonic there and I mostly agree with the sentiment you had.
I personally think the concept for Neo Metal Sonic could have been great and would develop the character a lot more but that's just not the case. Throughout the entire game, he's disguised as Eggman for some reason, I guess it was done so he could get all the characters' data but I feel like they could have executed the idea way better than just having him be Eggman all the time.
The only cool idea with the Eggman disguise thing was the concept of how Metal Sonic would suddenly treat the characters differently, this idea was only really done once in the Archie Comics but it's still a neat concept.
Either way in the end we don't even get to fight Neo Metal Sonic himself and instead we fight the giant Kaiju lizard thing he ends up turning into.
The only thing I truly actually like about the battle is the back and forth he has with Team Sonic in the final phase but other than that, it's pretty lackluster and it doesn't help that with the context of the games that came before this one, it just ends up feeling like the same final monster battle thing that was starting to get old at that point.
After the battle Metal Sonic is treated as a joke with him turning back to his old form and just passing out (or something) for some reason, they could have made him disappear after the battle and had a cliffhanger at the end teasing that he may come back or better yet they could have just killed him off as a whole because after this game is the point that he becomes a one-note uninteresting villain.
I really wish they did just kill him off because then maybe we could get to see another new Sonic robot, something we haven't gotten in years.
Like don't get me wrong I think Metal Sonic is cool but at the same time, it would be cool to see another Sonic robot other than him, maybe a mix between a battle model and speed model.
What I mean by that last statement for anyone not in the loop, you see there are two kinds of Sonic robots, one that is built as a battling machine that started with Sliver Sonic and ended with Mecha Sonic. (Who are both cyclops which was a genius joke if I may add)
The other type is the speed model which may have started with Proto Sliver Sonic (Game Gear) but mainly is associated with Metal Sonic and possibly Metal Knuckles but in his case he seems to be a lot more battle-oriented than Metal Sonic.
Anyways the difference between the two models is pretty self-explanatory, the battle models are slower and are bigger targets but they have higher defense and are a lot stronger which is evident by their curled-up ball forms alone.
And on the other hand, you have the speed models which you could already guess are the exact opposite of the battle models aka lower defense but higher agility.
With this being best shown in Metal Sonic's Generations battle where when you hit him, he dramatically crashes into the ground.
This scene alone is one of my favorite Metal Sonic moments because it shows that Metal Sonic is not built to be a battle robot, it helps that it doesn't ironically damage his character at all and is in line with his original concept.
Something I forgot to bring up before is that there is technically a third type of Sonic doppelganger robot but we haven't seen it in action for Sonic himself.
This third kind is just a pure copy of the character in question, this has only been done in-canon with two characters with them being Knuckles and Shadow.
Now both of these robots aren't 100% perfect with the Shadow Androids being weaker than Shadow himself and Mecha Knuckles being weirdly missed colored but still they are pretty close nonetheless.
And with that all established I want to bring the severely underused Sliver Sonic 2.0 to close this post off.
For years people were speculating about this random Sonic robot that can be seen in Eggman's base in the Sonic Adventure, in the same area you can also find Metal Sonic so it can be inferred that this robot was not meant to replace Metal but instead was meant to be its own thing.
It's theorized that it was meant to be the next line of Battle Sonic bots with a strong likeness to Sliver Sonic which is the reason why he was dubbed "Sliver Sonic 2.0" by fans.
Interestingly following up on that note, he's a lot slimmer than his predecessors with him looking eerily similar to Sonic himself even more so than Metal Sonic which could possibly mean that he was meant to be an in-between for the two models.
Now, something I should bring up before I go on is how he was created to be a Mechagodzilla reference and that might may have been all that he was ever meant to be.
Either way, he had a very interesting design that held a lot of potential but sadly he only ever appeared in Sonic Adventure but was used for the already mentioned, Mecha Knuckles.
And interestingly his power of disguising himself to appear to be the original Knuckles but still being noticeably not the real deal is something that Mechagodzilla also did with his Fake Godzilla form.
I guess what I'm getting at is that I wish they temporarily retired Metal Sonic for some games (before maybe coming back with an improved model later) and let Sliver Sonic 2.0 have some time to shine, I think they could have saved the whole disguising robot idea for 2.0 and gave Neo something more befitting to Metal Sonic for Neo.
Hell, they could do a "Sonic & Knuckles" thing with 2.0 and Mecha Knux, again there was so much potential here but they decided just to just stick with regular Metals which was a bad decision in my opinion.