Writers seem to think this, but it's not entirely correct. I know you didn't like The Wire - even though it is by far the Greatest Best Television Show God has given Man on the Face of the Earth - but there were several characters who were central to season 1 that did not make it past season 3.
I don't dislike
The Wire, it just didn't grab me. I'll try it again someday. But given the ratings on that show, it's not really parallel to a show on regular broadcast that is expected to grab a much larger audience than is ever expected on premium cable. When your audience is
paying to watch, you can afford to cater to a much smaller number of people. But if you need a larger audience because one pair of ad-watching eyeballs isn't worth all that much, you have to do the crowd-pleasing things.
So, here's a speculation that recently occurred to me: now that we know the specials may have gotten that way via genetic manipulation, doesn't it seem likely that the Petrellis were deliberately trying to create a power-sponge who would be the "ultimate hero" and Gabriel was the first attempt, which went horribly awry, so they tried again more successfully with Peter?
Wow, imagine when Sylar finds out from Ma Petrelli: "Well, Gabe, you came out screwed up so we tossed you out with the trash. But Perfect Peter,
him we kept."

I'm sure he'll take it with maturity and calm.
ETA input on the thread's most important question: I thought it was "booboo" as in what kids call their injuries. Much funnier that way in any case.
Booboo or boohoo, here's something else I remembered: it was in either the first episode or one of the very early ones where Mohinder searches Sylar's apartment and finds the secret room with FORGIVE ME scrawled in red paint or possibly blood. So why is the "boohoo" such a surprise if the very first thing we learned of Sylar was that he was remorseful?
Based on that message, I expected him to be a lot more conflicted than what we've seen from him to date. Future-Gabriel matches up with the guy who wrote that message much better than Sylar as he was presented in S2. So this isn't a retcon so much as just writers trying to get a handle on the character at long last.
If you could only keep five characters (they have to be regulars or strong semi regulars) with Kring filling in the cast with new characters--which five would you keep?
Angela, Nathan, Peter, Sylar and Matt. The first four because the Petrelli Family Circus is too good to lose - tho I'd hate to lose Claire - and Matt because I still think he's got interesting potential if they'd get to his story and move it forward at more than a tortoise-pace.