I'm going to be brutally honest. Nothing is going to bring back those days.
MCSG has been far too slow and far too reliant on their single good gamemode to ever return to the prominence it once had.
The game itself has evolved. Cheaters are no longer 12 year olds using forcefield and flyhacks ( though they still exist) - players, high up on this server's leaderboards, use ghostclients with more advanced, less detectable hacks. To combat something like that, you need an anticheat far stronger than what MCSG will develop. I'm sure it will ban the obvious hackers, but it won't deal with the ghostclienters. For that you have to have been like Badlion, developing a powerful anticheat before hack developers had even begun to think about bypassing something like that. MCSG needed to get out ahead of it. It didn't.
But it's not like the hacker issue is all of it. SG, as a gamemode, is dying. UHC, Skywars, and Pot PvP are in right now, and the current YouTube community reflects that. Youtubers who once relied on SG for all their videos are including more and more UHC and Skywars in their videos.
Then, of course, there's the community, once so small and peaceful. I remember going on AU and talking with the #1 player at the time, Elisha_Mutang, and teaming with him for a game. Everyone knew everyone, and things were nice.
Now I find a minimum of two teams a game, half the people are toxic, and targeting happens a lot.
Then there's the issue with glitches never getting fixed and the slow response of the dev team to, well, basically everything. A buggy, glitchy server used to cut it in the Minecraft serverscape, but other servers (Badlion, Kohi, and others) have raised the bar. MCSG slipped under it.
And that doesn't even begin to mention all the mistakes along the way. MCSG completely ignored their community by updating to 1.8, assuming all players were upset about nothing and that it would all boil over. It lost over half its playerbase and hasn't recovered. Don't even get me started on Solo SG, Battlegrounds, or "UHC Worlds".
Minecraft PvP has moved on. MCSG failed to keep up. Its time has passed.