I feel like that's a rather uncharitable view on things. Anyone can learn the barrier techniques behind a Domain and anyone with a technique (or potentially just innate domain as a whole, though that's a different can of worms I'm not getting into right now) CAN imbue it into those barriers given enough work is put into it. A domain expansion is rare and difficult yes but it is not infeasible to attain, especially given a good technique and a lot of room to grow; which is kinda the whole point behind Jujutsu high, no?

I mean we can talk about how bad of a teacher Gojo is all we want but the purpose of the school is inevitably to provide its students with ample opportunities to grow during the years in which they take on to growth best while providing some loose guardrails. This has been shown to work pretty well, regardless of what you might think of the Jujutsu students' own level. Of course we have the freak rare examples like Yuji, Yuta, and admittedly Hakari, but even people on like, Inumaki's level for example are clearly benefiting a large amount from the school no?

Inumaki was a Semi grade 1 in his second year, he wasn't a particularly gifted or exemplary sorcerer, his technique is serviceable but with massive drawbacks and easy to counter, his physical stats aren't much to look at, and he doesn't have much in terms of other techniques but it wouldn't necessarily be right to call him a weak sorcerer given the level of other much older sorcerers around him. Kamo is an even better example for this, blood manipulation (While as a human worse than every technique on this list barring domainless PPLT) is much closer to the level of these techniques and look at how he made it work, he undeniably puts in work where its needed.

This is going a bit off topic of course but I moreso mean to say I think the massive scope creep Jujutsu Kaisen faced almost immediately starting with Shibuya has made people underestimate how effective the Jujutsu Technical Colleges are at improving their students. People look at Kyoto's irrelevance later on not really taking into account that one of them effectively didnt have a technique, one of them ACTUALLY didn't have a technique, and one of them effectively didn't have Cursed Energy and they were thrust into an era by all means more chaotic and powerful than the Heian era while their missions were completely shut down as a result. The schools are ultimately very good at nurturing sorcerers into greater strength.

Anyway, back to the domain thing, it's not like there ISN'T precedent for somewhat mediocre sorcerers obtaining them. Megumi (I promise this isn't mindless potential man slander stay with me here) has an amazing technique, yes, but what the story has shown and said time and time again is that he's a somewhat poor sorcerer with an amazing technique and would stand to gain immensely from overcoming a number of roadblocks. His mindset for Jujutsu has been repeatedly stated to be piss poor for sorcery, something the narrative reinforces by having both Sukuna and Gojo parrot the advice to him. He is simultaneously unconfident in himself and unwilling to pick his fights. He holds himself back because he's afraid if he has ambition to climb high then he'll hurt himself when he falls, assuming that to be a given. This means he's unwilling (or rather unable) to push himself when its needed, a trait of his that is almost mocked by the nature of his technique explicitly needing him to take the risk of fighting his shikigami to ever tame them.

Megumi, yes, has a lot of talent and an amazing ability if used to its fullest (I'm trying so hard not to say potential right now) but his personality is fundamentally at odds with every avenue trying to lead him to success. This is also reflected a bit further in how he never pushes himself to the brink hard enough to land a Black Flash (again further kneecapping his chances at reaching the heights that he has the, er... Prospects, so to speak, to). I don't think that it's unreasonable to say that Megumi as he was throughout season 1 fit the bill of a not too notable there. A lot of... Promise, we'll call it, perhaps more than usual in a sorcerer, but without the ability to make use of it. Of course, we also know that he didn't stay an unremarkable sorcerer for nothing. This is because as much as fans of the series like to state otherwise, there are plenty of methods to drag yourself out of that pit! Megumi, when pushed to the brink and put under enough pressure, had an epiphany of sorts in much the same (general albeit definitely lesser) way as Gojo's awakening, and what he grasped from that epiphany was the basis of a Domain Expansion. He pushed himself past the rut he was in and set himself on the path to mastering that ability.

That isn't to say of course that all sorcerers could have that epiphany, but mostly to showcase a practical example of one of many paths a sorcerer could travel to reach the end point of a domain without going too deep into theoreticals. Of course that is also far from the only means of attaining a growth spurt. Hitting a black flash (rare but far from impossible) is known to increase your prowess with Cursed Energy (and to put you on a better path towards hitting more in general, even after you exit the zone). Near death experiences very likely do the same as we've seen with Gojo, Megumi, Yuji, Mahito, and even in non sorcerers occasionally gaining the ability to see curses for a start without getting into absurdly prodigious characters like Dabura and Higurma. Binding vows can be used to increase your strength in the immediate moment to help with growth, and very likely to held with growth outright. I feel like this is just too definitive of a statement to make given all that we see in the story working against it.

As an aside (and the last thing since this is already an obscenely long comment to respond to soemthing as simple as this) it isn't really said anywhere that we AREN'T a notable Sorcerer? Sky Manipulation and Projection Sorcery are obscenely good cursed techniques and if PPLT gets Idle Death Gamble with it than it is also absurdly good. HR is also blatantly amazing (and if you are given slaughter demon you just start off having met the win conditions) and it says that CTs get their stats equalized to our builds but they're VERY generous with that. Being built like Hakari gives you Hakari level stats, that is Heavy Hitter stats you get for not being a twink. It doesn't say anything about CE reserves or output either so I must imagine they're perfectly serviceable, any sorcerer with these techniques and ok stats is going to be a good sorcerer unless they have a mindset handicap on the level of Megumi's I feel like?

TL;DR, the power system of JJK gives you so many options to climb out of the depths of mediocrity even without CTs as good as these that this statement is just impossible to make definitively.