I want to share my thoughts on the matter.
I am actually fully content with the spot Kirby has: around the best of his current tier. (though I wouldn't mind seeing him in the bottom of Tier 4). I actually think that by design, Kirby has the tools to deal with pretty much any character. He has good tilts and aerials for combos, small frame which makes him tougher to hit, a great crouch to bypass projectiles from a lot of characters, kill throws, a combo throw, an Up B that creates a transcendant blade and projectile which can beat out appraoches and approach in turn, and an Inhale can put foes in a bad position or take their power which in some cases can completely change a matchup. On paper, Kirby would be able to handle the great majority, if not the entire cast.
It's just that right now, a lot of his tools just haven't been implemented well. His tilts and aerials would be great for combos and approach, but for some reason he has slower aerials than a lot characters of similar class. I mean for some bizarre reason, his N-Air comes out frame freaking 10. Same for his Up Air and F-Air. Compare this to other similar combo characters like Mario, Falcon, Yoshi, Pikachu, or Meta Knight who have aerials that are often done in half the frames or less. Notably mention goes to his D-Air which possibly his best move yet has 18 frames of startup. Considering how slow Kirby is in general mobility, it's strange why this move has so much startup, especially compared to Fox's D-Air. His only fast aerial is B-Air. His unusually slow aerials make it tough for him to keep. This makes hit harder for him to challenge foes in the air or make his strings last than it should be. I also feel that Kirby's Smashes are too slow as well. By themselves it may not seem slow, but consider Kirby's range and speed and you find that getting them to hit is a lot harder than it should be. The frame 14 Up Smash is especially notably as it's pretty much slower than almost every other Up Smash of its type, so slow that Kirby can't even drop shield and hit his opponent with it quickly enough if they hit his shield from above. You are also correct in that his Up Throw is a just too weak. Kirby can't setup a kill off his grabs so he needs a reliable kill throw but his Up Throw is very underpowered, not killing Mario until about 190% bar DI. (Which is funny though because it's still in Top 20 of strongest Up Throws). Considering its animation, it's rather abusrd that characters like R.O.B and Marth have stronger Up Throws.
I personally think that the biggest issue with Kirby's whole neutral though is Final Cutter. I am surprised by how many overlook what a crucial move this could be. I think if Final Cutter had the startup of Upper Cutter and way less endlag with the range it had from Brawl, it would single handledly make Kirby's neutral solid. Think of all the uses he'd get out of it. The blade would allow him to beat out disjoints and breakable projectiles from a safe distance, even beating things like Yoshi eggs. It'd give him a nice landing option and a combo breaker. It'd become a powerful anti air that would make his matchups against many characters like Peach and Ness improve greatly, and it'd give him a form of approach and zoning. But sadly, the move has massive startup and endlag and got its range neutered. This is the attack I'd love to see fixed most as it would single handedly make so many his harder matchups even or tip in his favor completely.
As for the bit on those top and high tiers, I am reminded of a post Emblem Lord made a while back in regards to how the characters that should beat Ryu handily falter due to being underpowered. I think that by innate design, Kirby beats all those characters. But his tools are too underpowered at the moment for him to properly push those advantages. While I have my own opinions on the matchups individually, I due agree that Kirby isn't QUITE at anti-meta level yet. Getting there but not quite.
Having said all this though, I think Kirby arguably has the most potential of all the characters in his tier and below. Right now I personally feel he's about 31-37. Nice but lacking. But with the correct buffs to his frame data, Final Cutter, and throws, I could easily seem becoming a solid high tier. Thus while I personally have to agree that I don't think Kirby's quite ready to jump to tier 4, he certainly deserves to stay in his current spot as I personally that compared to a lot of the characters around him, if his design was implemented as well as it could reasonably be, he'd have the best time handling the overall cast.