Honestly I feel like the ‘threat’ of locke is overly exaggerated by the fanbase. From what little we know of his role, it sounds like he’s merely playing second fiddle to Chief, and his role is more to tell a story you really can’t if you constantly focus on chief.
As for him being a match for Chief, I’m really tired of the building notion over the years that chief is the best spartan in terms of combat capability. In FoR and even what I recall of First Strike, the only thing he notably seems to excel at is leadership ability. This is why others are attached to him. Blue team alone has other areas that they excell at, with Fred ‘chosing’ to be 2nd best, and Linda pulling off ridiculous sniper antics. Unfortunately, halo has not been a series of games that have actually showcased what makes him great.
Instead, its relied on his luck. As we all know, Chief has saved the human race and even the galaxy from a number of threats time and time again. But part of his luck seems to be his ability to simply be there to do it.
At Reach’s fall, which is the first time spartans have shields en-mass (and not just random testing in halo wars), Chief’s the only able one who makes it to Installation 04 for his first big act of heroics. Where are the other spartans? Ordered down to defend a doomed planet from the elite’s pants-on-head plan given their super ship. The other two were shot off into space, and unloaded on by a needler. Neither event was the fault of the spartan themselves, merely the circumstances they ended up in. So, chief goes to I04 and gets to be a hero largely because he wasn’t sent on the suicide mission. There’s no indication the other spartans couldn’t have done similar things as him on I04, as they are never even given a chance to try.
After the destruction of I04, he is reuinted with Blue team, and his most notable combat acts in that book are struggling with an elite and a brute, whereas Linda is sniping people out of banshees while upside down hanging from a cord.
So, then halo 2 rolls around. Where is blue team? Off being questioned by ONI about stuff. Once again, not their fault they aren’t there on Cairo station to fight alongside chief, and end up on the ship that chases regret. Once again, other spartans aren’t given a chance through no fault of their own.
Then halo 3 rolls around. Where is blue team? Inside a forerunner shield world due to Ghosts of Onyx. Honestly chief seems toned down in ‘superness’ from h2 (where he could have completed RED FLAG on his own) with elites effectively saving earth from the flood, and the arbiter being there to help him on a lot of key operations. Johnson repeatedly failing seems to be a plot device used to make chief feel needed.
It’s halo 4 that we finally see some other spartans in game, and they are the rather ‘new’ S-IV’s lead by a cowardly captain and a commander who loves just charging in and insulting everything (you know…I’m kind of curious how her and Del Rio ever got along). Sadly blue team is just kind of in limbo, and brought back in a comic. In the comic, the team seems rather equal again. Chief does get a stab at the didact’s eye, but thats because the didact has a personal grudge on chief and couldn’t just kill him normally, but had to bond-villain it up.
That all being said, the ‘replacement plot’ as has been brought into halo is usually one in media where the hero proves they cannot be replaced, not that they are actually replaced.