This article doesn't seem to say, so, how many of these search engines use their own index and don't "borrow" one from another larger engine? DDG uses Bing and Yandex, for example.
But hey, everything is "fine" as longs as the Web keeps a bunch of developers employed with six-digit salaries. They will put up with any amount of accidental complexity and ignore any effects on future innovation as long as their jobs are secure. (And those jobs are more secure than ever because you need ever increasing number of specialized professionals to keep the increasingly complex technology stacks operational.)
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* One exception I know of: Web Archive. But their coverage is pretty spotty and they aren't strictly speaking a search engine. Still, it's an awesome effort. At lease someone tries to swim against the tide.