How do you even define the ideal meritocratic hypothetical scenario (the details mentioned seem arbitrary and without motivating principle-though some sound pleasant), and what are your actual arguments that this particular method of governance is better than Democracy?
I think you seem settled on your mind that anything different is a threat to Democracy, almost as if the capitalism as we know is the only way that can support Democracy.
The "details" (they are not actually deep) I've mentioned seemed arbritary but they are general agreement towards many people of what is a minimum of meritocracy at least for those who honestly wants it. Equal opportunity for every children is already a basic principle. Providing equal education and health at least for the children is quite basic. Limiting wealth passing (the heritages) is also basic as well. And so on for the rest of what I said.
Meritocracy does not necessarily kills Democracy. The principle that i've said didn't even hit politics. It is not as if we elect our bosses and who gets promoted anyway... Selecting by job performance does not affect politics, none of what I said takes democracy out.
The implementations of meritocracy on politics are varied and debatable. Can go to basically keep the democracy system, I've seen (that's many years, so no links) some people talking about making more basic requisites and preparations and a minimal qualification for the politics (which is sort of desirable to have more proper and prepared polticians), and then the possibility of politicians being elected based on merit and results rather than being elected by people.
As I've said, actually my opinion is more against the monopoly of capitalism rather than defending meritocracy by itself. My opinion would be to actually test and evalute some meritocracy variants to see what is best. I would put my bet that having a higher qualification for politicians than it is today - and a quite high bar for politicans like president - while still allowing people to vote for the candidates would be better than rather the indications of the parties.
My own ideal scenario would be to fit at least the basics, but a little bit beyond. I would say children and teens at least rather than children only should have equal access to education and health for free in the same leveled of quality without any privilige. Then, on the university or perhaps a little bit earlier, yound adults would start to being selected to better universities according to their own results and merit on exams. After that, on their first job, they would be evaluated in a job performance that is the most objectively as feasible, which would weight on their promotions (and not simply as the boss likes and wishes as it is these days). And all with actual fullfilment of some minimal level of ethics. And for the wealth passing/heritage I would be compeletely in favor of a million or 100k dollars limit (since a million is already quite much). That value is still arbritary, it wouldn't be bad if it were weighted based on votes.
Of course there would be much details but I am not sure if I am willing to write a lot of them taking into account that my hopes for this to happen is quite low, even though I am aware the forum and thread are more of a discussion on the hypothetics scenario. On personality, one of the old sources (I remember the site to be down) suggested split classes by MBTI, which is not adequate since the MBTI doesn't have a good test-retest rates and we can't just have 16 classes in every school, but a more proper system that sorts students as either temperament (specially on a more stable version) but another good idea would be to split in a few learning styles with teacher with each class being focused on a learning style.
As for the arguments with it against capitalism, well, I'd say that meritocracy seems more promising in terms of efficiency and fairness. Efficiency in terms that, instead of a lot of arbritary networking, where capitalism basically works to maximize the freedom of the bosses in practice (boss hires almost whoever they want, except in some very specific professions where a license is required), allowing them to put family members, doing incredibly subjective hiring selection, to instead make a selection that is based more on actual competency. I'd also in favor of a better support for selection for professions and to a free search and consultation of vocations and professions. Putting the people closer to what they are at best would be more efficient and thus it would likely to work better and worth of a good shot. The fairness aspects weight as well.. capitalism throw people at poverty quite arbritarily.
I do have one less hypothetical thing to say, though. What I've noticed after going to Uni, school, work, etc.. is that the most meritocratic and fair system I've ever saw are actually in some games I've played. Specially on the last decade and on the 00's, a lot of games were fairly meritocratic even in a deep sense. One of the players on the high ranks are in fact good at game and in fact not only they are reliably expected to beat you with the same tools rather than with arbritary advantages but it is quite reliable that they are good and competent at the game. The same cannot be said to management, president, bosses, etc.. on where we live, even though we cannot irl just throw them this fact. In one of the older games, I've could even tell that a great player was in fact a great player by evaluating their statistics without ever seeing them ever playing, did it a few dozen times without making err in a single one - if I said the person was great, some of my friends who met the person on the game did confirm, even though some of them disliked my methods. We just don't have it on the professional lines. Doctors around the world influenced by Trump, even with their licenses, are going to prescribe you chloroquine. Some bosses are not really much good but, well, somehow they make it in there. Some others are full of wasted potential and that goes on. On games with proper fairness, nice players doesn't need to clean the boots of any developer or any bosses at all and neither to rely on luck.