(sorry, this got long)
A single party system sounds very scary, because how can you vote them out? But... that's just not how China's system works. I'm not an expert on China at all, and I don't mean to claim to be. I'm just an internet dweeb, but I'd still like to try to explain what I mean:
Within a party like China's, there isn't just bland homogenous unity. There are factions, with histories and ideological alignments and family connections, aspirations, and so on. Xi Jinping's ascendance was part of a long political game that came along with specific groups, ideas, and policies to the forefront of Chinese political power for a period, and it was supported by the prevailing milieu within the party, and could not have happened without them.
Over time, that milieu changes, and so do the party's goals, priorities, and actions. That happens as the course of external events and affairs, internal politics, economics, public sentiment, fortunes, etc. all unfold to shape it. Jinping has acted to maintain his power and continue his own work and vision, which appears to still be supported by the political powers that surround him. Changing their Constitution (to allow further terms) is a legal action, and they followed a process to do so. It would have required significant political capital to see that process through. It was hardly flouted, even if we balk at the result.
Fundamentally, what is done by the Chinese government is always simply work to preserve and extend: China's sovereign power, the well-being of its people, and the wealth of its ruling class. This is basically true of all nation states.
In America, it is difficult to change the Constitution. Maybe more difficult than in China; constitutions have different levels of significance in different places. But there is similarly a process for it that has been executed many times.
If someone really needs to, they can try to accrue and expend the required political capital to get an amendment done. There's been talk that the Koch brothers and some associated folks have been working on that for years[1]. In the meantime, Presidents, at the behest of whomever they are politically beholden to, work hard to get Judges in place to interpret laws however they may need, and those interpretations vary wildly.
Meanwhile, the two parties in America change as their cultural milieu forged by similarly shifting economic, political, etc. forces unfolds. They consistently line their own and their friends' pockets, start wars, create secret courts, surveil people, and so on.
So how different is having Just Two Parties who take turns depending on a very big vote, but that think in and act in many fundamentally similar ways... from Just One Party with an ever-changing internal makeup, whose leadership takes turns depending on a smaller vote? Ultimately, the American government still and always works to extend America's sovereign power, the well-being of its people, and the wealth of its ruling class.
To repeat myself a little from another post, I'm personally much happier living in a Western-style democracy than a Chinese-style system, where we at least maintain the illusion of real popular control. But the changes we can really effect as individuals, and the power we really have, in either system, are quite limited. While different tools are employed, the aims, the justifications, and the results are broadly similar in the end.
Perhaps you think the United States differs fundamentally in that it can't or won't use violence or underhanded political tactics against its own citizens, to keep the dissidents it really fears down? Read about Sandra Bland, Fred Hampton, or Martin Luther King. There are many, many examples here -- the mask of just benevolence slips off pretty easily.
What I'm ultimately saying is that, if the United States government needed to, it could and likely would use the same tactics the Chinese government does. It already does from time to time in limited contexts.
We, the people, get to choose as long as it's between Harvard and Yale, and if we got really dangerous and started to forcefully demand, say... anything really different? The string of unfortunate coincidences would come out first, and if we somehow kept our momentum up past that, well, then the gloves would come off for a while, until we quieted down.
We feel good about our worldview precisely because we are able to imagine that those gloves cannot come off, or that they wouldn't ever unless it was really justified, and then we'd be on the right side of it, right? But in reality they don't serve 'justice'. They serve the extension of America's sovereign power, the "well-being" of its people, and the wealth of its ruling class.
All that to say, I still feel like it is pretty similar.
1 - https://www.salon.com/2018/08/10/the-biggest-threat-to-our-d... (I just gogoled 'koch constitutional amendment' here, there may be better sources)
(edit: I corrected Fred Hampton's name from my typo'd Thompson.)