The definition of the word "success" is "the accomplishment of one's goals". For a group to be successful, its members need to agree on the goal. Failure to agree to a goal means that, by definition, the group can not be successful.
For example, if half of the Vikings defense wants to stop the other team from scoring, while the other half wants to let the other team score, then the Vikings defense can not be successful.
Even if a group of people can agree on the goal, they also need to agree on the values via which the goal can be accomplished, in order to be successful. If everyone in a company agrees that said company should manufacture, market, and sell widgets, but half of the people think this can be accomplished exclusively by having parties all day, then the group can not be successful.
It is my contention that for groups that can not meet these two criteria, their members should successively secede until the groups are small enough for the criteria to be met.
It is so obvious that shared goals and values-- so, the very opposite of diversity-- are strengths, that you can even look at the behavior of people who claim "diversity is a strength" for proof.
When HR departments of companies are hiring people, they ask lots of questions to determine if the person would be a good "culture fit" for the company-- meaning, does the person share the company's goals and values? CEOs of big "pro-diversity" companies even actively weed out people who share different values, such as the James Damore firing from Google.
When people sit together for lunch, have you ever noticed that the women tend to sit together at one table, the men at another, the Indian people at another, the black people at another, and so on? That's because people recognize that their chance of success in life-- for example, their odds of having an enjoyable lunch-- is accomplished by associating with people who share their goals and values.
If this stuff is so obvious, then how does the "diversity is a strength" mantra propagate?
At the top of the power structure, Western politicians benefit from mass immigration (diversity) of third-world people, because those people will enter the country and vote for government "programs" which will grant the politicians and their coalition more power. Additionally, the mass immigration will cause social friction, which the politicians can leverage via the age-old "divide and conquer" strategy.
CEOs of companies love mass immigration (diversity) because it gives them access to mass-scale, cheap wage labor. In particular, this is why tech companies specifically push for "diversity"-- much of their labor force originates in countries that have much lower salary expectations.
As for your common-day "progressive", they are taught the monkey, bang-stones-and-chant "diversity is a strength" mantra from the cradle. Most people take for granted these kinds of assumptions unquestioningly when they are brainwashed as young children. You can see this in the blind "cult of personality" worship visible in places like North Korea.