In the last 30-40 years most widespread fear of wizards was destroyed - fear to get a Squib in their family. That fear was, is and will be the most foundation for all "pure-blood" theories - based on anxiety that "breed with Muggles" and even communications with them could be a reason to decrease "Muggle-lover`s" magic abilities. Of course, there is no slightest sign of truth in these "conspiracy theories" at all - magic comes through the world as a great flow, and human beings get it as accidentally as possible. But Wizards do not like to confess, that they are exposed to aberrations as well as Muggles.
So, why this fear is destroyed? Squibs are still born sometimes. But - what do parents want for their children? Health, life in peace, enough wealth for decent life and self-fulfillment. Usually, wizards tend to believe that all that achievements directly depend on magic abilities. All careers in wizard world depend on magic potential of applicants - but the Big World is wider than wizard part.
A lot of Squibs do great career between Muggles, for example - Steve Jobs, Matthew McConaughey, Lindsey Stirling, Ilya Kabakow etc. Even "pure-blood" Wizard families, that consider having Squibs between their relatives as deepest shame, invest in their Muggle-businesses - as we could see in case of MalfoyStanley, biggest Wizard shareholder of Apple Inc. Their success brings not only a hope for those wizards, which abilities are not enough for success in wizard community, but also a little bit shake the confidence of all wizard youth in necessity of magic itself.
Even if weak, almost imperceptible magic gives a lot of advantages in Muggle world - so, why do you need more of it? Some restrictions in use of magic in Muggle life that created by International Statute of Wizarding Secrecy help to keep young wizards on study of magic, but it is not enough.
From the other side, relations between Muggle parents and Muggle-born wizard children aren`t nice so far. Magic abilities create tensions - parents hope that heirs will run their businesses, or will become doctors (as Muggles names Healers), engineers, police officers etc. Wizard children usually choose magic, and leave Muggle world, world of their parents forever, abandon all that hopes.
But also some young Muggle-born wizards choose Oxford or Cambridge for their education and throw Hogwarts letters out.
These days, despite of all statistic data came from Ministry of Magic, we can conclude, that amount of young Muggle-born wizards, who decline invitation to Wizard World grows from year to year.
It was absolutely impossible and unimaginable 40-50 years ago, but it`s true.
It depends on multiple factors. Some Muggle inventions, such as Internet, cellphones, social networks, airplanes, robotics and biogenetics etc., do Muggle world more attractive than before.
Muggle world can provide more possibilities for self-fulfillment and for communication, affairs and love, even for love!
As a result, Magic is loosing to Technology competing for youth.
We, Wizards, have to find the will to carry on, as one famous Squib sung. We need to improve our magic, to create newest educational programs, to invent new spells and charms that will be more powerful and accurate, more useful and comfortable, more long-range and wide-angle.
And we should understand that this competition will exist forever.